《越南大戰4》於2002年在Neo Geo MVS街機平台推出，由韓國Mega Enterprise與Noise Factory共同開發，是系列首次由非SNK本家製作的主要續作。雖然在外包的背景下問世，但仍延續系列一貫的橫向卷軸射擊節奏，加入全新角色、新武器、新載具以及創新的「Metalish」得分系統，使整體呈現出既熟悉又帶有過渡意味的風格。

故事設定於《越南大戰3》後一年，全球軍事網路突然遭受名為「White Baby（白色寶寶）」的神祕病毒威脅。這種病毒宣稱能突破任何軍事安全系統，其來歷則指向自稱「被神所愛者」的恐怖組織——阿瑪迪斯（Amadeus）。

面對這場跨國危機，各國立即召開緊急會議，一方面強化軍事設施防護，另一方面則決定派出特種部隊PF（Peregrine Falcons）追查阿瑪迪斯並摧毀病毒源頭。然而，PF 的兩名主力成員Tarma與Eri因負責護衛研發疫苗的科學家而無法上前線，使得主角Marco Rossi與Fio Germi扛起核心任務，並由新成員Trevor Spacey和Nadia Cassel加入行動。

在任務開始前的衛星影像中，PF小隊意外看見一張令所有人驚愕的臉——莫登將軍。這位宿敵的再度現身，使整起病毒事件更加撲朔迷離。隨著玩家推進關卡，阿瑪迪斯的真相逐漸揭露。最終關卡中，會抵達一座秘密地下工廠，驚見大量莫登將軍的機械複製人被量產，就連系列知名反派Allen O’Neil也被揭露為機械假體。

最Boss被稱為「Doctor」的男子。他操縱多台巨型戰鬥機械與玩家交戰，但最終反被自己引發的裝置吞噬而毀滅。若成功逃離最終自爆區域，結局將出現主角團大吃慶功宴的畫面；若未逃出，則會在醫院全身繃帶地接受Tarma與Eri送來的慰問食物，形成詼諧的另類結尾。而在片尾最後，一台黑暗中亮起的電腦螢幕仍在向未知端點傳輸資料，象徵阿瑪迪斯的殘餘勢力仍潛伏於某處，使故事留下一絲未盡的餘韻。

核心操作延續系列傳統：射擊、跳躍、投彈、搭乘載具、拯救俘虜等經典要素一應俱全，並保持街機特有的速度感與爆炸張力。本作的最大革新，是新增「Metalish System」——一種以時間壓力為核心、強化分數與節奏競速性的全新機制。

在關卡中拾取「勳章徽章（Emblem）」便會啟動Metalish計時條。角色會在短時間內獲得無敵狀態，並透過擊殺敵人累積勳章。勳章的顏色與數量將形塑一個「得分階級」，階級越高，關卡結算時獲得的額外獎勵就越豐厚。若在計時結束前未達到勳章標準，階級便會下降；若在最高階級但勳章不足，則會直接喪失加分。這個系統迫使玩家必須在短時間內進行更高風險的攻擊，使得傳統的清場模式轉化為更注重節奏效率與路線掌握的玩法。

儘管沿用系列武器架構，本作仍加入數種新要素以增加變化，包括火力更強的「雙機槍（2H）」，雖然彈數有限但射速驚人，成為玩家珍貴的高火力選擇。載具方面，不僅能奪取Bradley戰車，更可搭乘獨特的Metal Crow與Crawler機械載具。最具特色的，是能以叉子刺穿敵人的Forklift Truck（叉車），既搞怪又具破壞力。此外，還設計「猴子變身」機制。若被特定科學家射出的化學彈擊中，角色會變成跳躍力強、能吊掛天花板管線的長臂猿，但攻擊力大幅下降；若再次被命中則會當場死亡。這種變身既是障礙也具有策略性，成為本作獨特的小型變化點。

關卡分布從城市街道、沙漠地帶到地下設施，各場景的節奏緊湊、敵兵密度高，符合系列的街機爆發風格。相較於《3》與《X》中大量使用的變異生物、外星元素，本作顯得更貼近初代的軍事對軍事設定。這種回歸雖使本作在視覺上較保守，但也讓部分玩家認為創意不足、特色不如前作強烈。

自發售以來的評價相當兩極。對許多粉絲而言，遊戲仍保有系列應有的節奏快感、華麗爆炸與操作爽度，Metalish系統也為高階玩家帶來全新挑戰。然而，它同時也背負著SNK倒閉後外包開發的背景，缺少部分前作獨有的幽默感與美術靈魂，並因素材重複與關卡保守而遭受批評。整體而言，《越南大戰4》在當年街機市場的表現穩定但稱不上亮眼，被視為SNK Playmore重掌系列前的過渡之作。儘管如此，它仍保留《越南大戰》最重要的核心精神——高速、瘋狂、爽快、充滿爆炸的射擊節奏，同時也為後續的《5》與《6》奠定技術與設計基礎，使其在系列中占有不可忽視的一席地位。

Metal Slug 4, released in 2002 for the Neo Geo MVS arcade platform, was developed by Korea’s Mega Enterprise and Noise Factory. It marked the first mainline entry in the series not produced by SNK itself. Although created under an outsourced development model, the game continued the familiar side-scrolling run-and-gun formula, while introducing new characters, weapons, vehicles, and the innovative Metalish scoring system. The result is a title that feels both recognizable and transitional in tone.

The story is set one year after the events of Metal Slug 3. A mysterious computer virus known as “White Baby” suddenly infiltrates global military networks. The virus claims the ability to breach any national defense system, and its origins point to a terrorist organization called Amadeus, whose members proclaim themselves “those loved by God.”

In response to this worldwide crisis, nations hold an emergency summit. While strengthening their military cyber-defenses, they also deploy the elite Peregrine Falcons (PF) to locate and eliminate the virus at its source. However, two key PF operatives, Tarma and Eri, are unavailable due to their assignment protecting scientists developing an antivirus. Thus, the bulk of the mission falls to Marco Rossi and Fio Germi, joined by newcomers Trevor Spacey and Nadia Cassel.

Before the operation begins, satellite imagery reveals a face the team never expected to see again: General Morden. His sudden reappearance casts a deeper shadow over the virus incident. As players push through the stages, the truth behind Amadeus gradually surfaces. In the final area, the team discovers a secret underground factory mass-producing mechanical clones of Morden. Even the fan-favorite recurring foe Allen O’Neil is revealed to be a mechanical duplicate.

The final boss is a man known only as “Doctor.” He commands several giant combat machines against the player, but ultimately perishes when his own device consumes him. If the player escapes the final area before it explodes, the game ends with the heroes enjoying a lavish victory feast. If not, the ending shifts to a comedic hospital scene, with the protagonists wrapped in bandages and receiving consolation food from Tarma and Eri. In the last moments of the ending, a lone computer screen glows in the dark, transmitting data to an unknown destination—hinting that remnants of Amadeus still linger and leaving the story with a subtle sense of unease.

The gameplay retains the core elements of the series: shooting, jumping, tossing grenades, riding vehicles, and rescuing hostages. Explosive action and arcade-style pacing remain at the forefront. The major innovation is the Metalish System, a new mechanic built around time pressure and score optimization. When players pick up an Emblem during a mission, a Metalish timer activates. For a brief period, the character becomes invincible and can collect medals by defeating enemies. The color and quantity of these medals determine a score tier—the higher the tier, the more bonus points awarded at the end of the stage. Failing to meet the tier’s requirement before the timer runs out lowers the score level, and reaching the top tier without enough medals causes all potential bonuses to be lost. This system encourages aggressive, high-risk play and transforms the usual “clear the screen” rhythm into one focused on efficient routing and tempo control.

While building on the series’ established arsenal, the game adds several new features. The most notable weapon is the high-powered Double Heavy Machine Gun (2H), rare but extremely effective due to its rapid fire. Vehicles also expand in variety: players can commandeer the Bradley tank, as well as ride the distinctive Metal Crow and Crawler machines. Perhaps the quirkiest addition is the Forklift Truck, capable of skewering enemies with its lifting fork while allowing the rider to continue firing personal weapons.

A unique transformation mechanic also appears. If struck by a scientist’s special chemical shot, the player character transforms into a long-armed monkey with higher jump capability and the ability to cling to overhead pipes, though with notably reduced attack power. A second hit results in instant death. This transformation, both humorous and strategic, adds a small but memorable twist to gameplay.

The game’s stages range from urban streets and desert landscapes to industrial and underground facilities. Each maintains a brisk pace and dense enemy placement, emphasizing the franchise’s trademark arcade explosiveness. Compared to Metal Slug 3 and X, which featured aliens and mutated organisms, Metal Slug 4 returns to a more grounded military-versus-military theme reminiscent of the early entries. This aesthetic shift, while faithful in one sense, led some players to feel the game lacked the creativity and spectacle of its predecessors.

Reception after release was notably mixed. Many longtime fans praised the game’s preserved pacing, solid controls, and explosive entertainment value. The Metalish system was particularly appreciated by high-level players seeking new challenges. However, the outsourced development and absence of SNK’s signature stylistic charm led to criticism. Recycled assets, conservative stage design, and reduced humor contributed to perceptions that the game lacked the “soul” of earlier installments.

Overall, Metal Slug 4 performed respectably but not spectacularly in the arcade market and is widely regarded as a transitional entry released during SNK’s period of restructuring. Despite its shortcomings, it upholds the essential spirit of the franchise: fast, chaotic, satisfying run-and-gun action. It also paved the way for the development of Metal Slug 5 and 6, making it an important, if imperfect, link in the series’ evolution.

Metal Slug 5, released in 2003 for the Neo-Geo MVS arcade platform and co-developed by Noise Factory and SNK Playmore, serves as the final mainline entry of the series on the Neo Geo hardware. Since its predecessor, Metal Slug 4, was outsourced and received mixed reception following SNK’s bankruptcy, Metal Slug 5 was widely regarded as a “return to form” after SNK Playmore regained control of the franchise. In terms of visual style, enemy design, and stage pacing, the game realigns itself more closely with the original spirit of the series. Although it does not reach the massive scale of Metal Slug 3, it demonstrates far greater consistency and reestablishes the core arcade intensity that defines the franchise.

The story takes place amid escalating global conflict, with the PF Squad working on a new generation of Metal Slug vehicles, hoping to strengthen their firepower against rising turmoil. Suddenly, the government’s SV research facility is assaulted by an unknown armed group, and a disc containing sensitive Metal Slug secrets is stolen. The data includes detailed specifications of all Slug models, PF Squad combat analytics, and studies on Morden’s weaponry—information so crucial that, if captured by hostile forces, it would completely expose the government army’s weaknesses. As the situation becomes critical, Marco and Tarma receive an emergency order to retrieve the disc at all costs.

At the same time, Eri and Fio—members of the elite intelligence unit “Sparrows”—are tracking a mysterious armed syndicate known as the Ptolemaic Army, an organization that plunders ancient cultural sites under the guise of archaeology. Their motives are unclear, and the group operates shrouded in secrecy. The two agents trace them to the ancient ruins known as the “Corridor of Fire,” but when they attempt to reclaim stolen artifacts, they are repelled by native defenders and a massive, deity-like Metal Slug revered within the ruins. As the two strands of investigation converge, the PF Squad and Sparrows confirm that the Ptolemaic Army orchestrated both the SV facility attack and the theft of the classified disc. The group also appears to wield formidable power linked to an ancient mechanical civilization, surpassing contemporary military technology.

Because the disc’s contents are extraordinarily dangerous, a full-scale assault could jeopardize government forces even further. Headquarters therefore initiates a small, elite infiltration mission. Marco, Tarma, Eri, and Fio regroup to venture deep into uncharted ruins and enemy strongholds, aiming to retrieve the disc and eradicate the Ptolemaic Army.

In gameplay, Metal Slug 5 preserves its run-and-gun foundation but delivers a sharper, more responsive feel. Its most distinctive innovation is the introduction of the slide maneuver, which is performed by pressing jump while crouching. This new action allows players to quickly dodge enemy fire, pass through low terrain, and maintain close-range movement flow, making overall combat smoother and more dynamic. Weapon balance is also adjusted: the Dual Machine Guns now hold 300 rounds instead of 200, providing higher burst firepower. In contrast, the controversial Metalish System from Metal Slug 4 has been entirely removed, returning the focus to pure arcade gunplay and momentum. Transformation mechanics are nearly eliminated as well, with only the classic “Fat” mode remaining—and appearing solely in the first mission—bringing the game’s identity closer to the early entries.

Stage design in Metal Slug 5 combines a return to military themes with distinctive elements of ancient religion and mechanical civilization. From urban chases and jungle infiltrations to the dark corridors of lost ruins, the Ptolemaic Army’s fusion of archaeological motifs and ancient machinery lends the game a tone that is both mysterious and dangerous. The final stage in particular transports the player into a colossal temple-like arena, culminating in a battle against a massive, enigmatic construct straddling the boundary between creature and machine—visually setting the game apart from the alien and mutant themes seen in earlier titles.

Upon release, Metal Slug 5 received stable, generally positive reviews. Many arcade players regarded it as more polished and more authentically Metal Slug than 4. The addition of the slide action improved mobility, enemies and environments reflected the restored SNK art style, and combat pacing regained its tight, explosive rhythm. However, due to limited development resources, the game offers fewer stages and relatively short total playtime, with some repeated assets. The final boss encounter also lacks the grandiose spectacle of the series’ peak moments. Even so, its refined gameplay, precise controls, and stylistic realignment helped the game maintain strong popularity through later releases on Xbox, PS2, PC, and modern platforms, making it one of the most frequently revisited entries of the series’ later era.

Overall, Metal Slug 5 is not as monumental as 3, nor as transitional as 4, but instead stands as a concentrated, mature distillation of the series’ classic flavor. With military action at its core, fused with ancient religious motifs and mechanical civilization, and energized by the fast, seamless slide mechanic, the game achieves impressive fluidity despite its modest scale. Though not the series’ most ambitious entry, its solid execution and clear identity secure its important and unmistakable place in the history of Metal Slug.