越南大戰4由Mega Enterprise開發，於2002年在NEO GEO推出，之後移植至 PlayStation 2，是系列史上最具轉折意義的一作。當時原開發商SNK正經歷重組，開發權交由Mega Enterprise，因此在整體風格上與前三代的「科幻大戰」系列大相徑庭，回歸早期的「軍事反恐」路線，並進行首次角色大換血，試圖在經典框架中尋找新的突破。

故事背景設定在全球遭遇新型電腦病毒「白色嬰兒（White Baby）」威脅之下，這套病毒能癱瘓世界各國的軍事指揮系統。情報部門調查後發現，幕後黑手是名為「阿瑪迪斯（Amadeus）」的神秘恐怖組織，他們甚至與舊敵摩登將軍的反叛軍秘密結盟。為了阻止世界毀滅，正規軍派出老牌戰將 馬可（Marco） 與 菲歐（Fio），並帶領兩位新成員——擁有白髮的熱血青年 崔佛（Trevor） 與格鬥技精湛的少女 納狄雅（Nadia），展開跨越雪地、城市與摩天大樓的神秘反擊戰。

在操作與系統設計上，引入全新的 「金屬彈頭勳章系統（Metallish System）」。在撿取特定加分勳章後，畫面會出現倒數計時，在時間內連續擊殺敵人可成倍累積分數，並在關卡結束後獲得極高評價。這項設計要求追求高分的玩家必須重新規劃行進節奏，增加遊戲挑戰性與競技感。

在 「載具劫持」 上也展現創意。可以奪取敵方的吉普車、戰鬥叉車甚至雙腳步行機甲，將敵方武裝反過來壓制對手。雖然關卡取消前作的複雜分支路線，但場景中大量陷阱、自動機關與科幻實驗室設計，依然維持2D射擊的爽快感與緊張節奏。

市場評價呈現兩極分化。正面評價高度肯定其 「回歸傳統軍事風」 的嘗試，認為緊湊的關卡節奏重現初代硬派戰場氛圍，新角色崔佛與納狄雅也為系列注入新鮮血液。PS2 移植版提供穩定執行與額外練習模式，受到家用主機玩家好評。

負面評價則集中於 「像素資源重複利用」。由於開發團隊變更，大量借用前三代的背景素材與敵人模組，部分老玩家認為缺乏原創性。此外，相較於第三代的宏大宇宙觀，本作關卡規模偏小，缺乏震撼視覺效果。

儘管如此，《越南大戰 4》依然是一部合格且爽快度滿點的續作，成功守住SNK轉型期的金字招牌，使熱愛2D射擊的玩家在新世代中依然能享受炸裂全場的破壞樂趣。

Developed by Mega Enterprise and released in 2002 on the NEO GEO, later ported to the PlayStation 2, Metal Slug 4 is one of the most pivotal entries in the series. At the time, the original developer SNK was undergoing restructuring, and development responsibilities were handed to Mega Enterprise. As a result, the overall style diverged significantly from the “sci-fi warfare” theme of the first three installments, returning instead to the traditional “military anti-terror” route. The roster was completely overhauled for the first time, aiming to explore new breakthroughs within the classic framework.

The story is set against the backdrop of a global threat from a new computer virus called “White Baby,” which can paralyze military command systems worldwide. Investigations by intelligence agencies reveal that the mysterious terrorist organization “Amadeus” is behind the outbreak, secretly allied with the rebel forces of former enemy General Morden. To prevent worldwide catastrophe, the regular army dispatches veteran fighters Marco and Fio, accompanied by two new members—white-haired hot-blooded young man Trevor and martial arts-skilled young woman Nadia—to launch a daring counterattack across snowy landscapes, urban streets, and towering skyscrapers.

In terms of gameplay and systems, Metal Slug 4 introduced the new “Metallish System”. When players collect specific bonus medals, a countdown appears on-screen. Successfully defeating enemies consecutively within the time limit multiplies the score, earning high ratings at the end of the level. This system requires hardcore players chasing high scores to carefully plan their pace, adding a new layer of challenge and competitive depth.

The game also innovated in “vehicle hijacking.” Unlike previous entries where players could only pilot their own Slug tanks, here players can seize enemy jeeps, combat forklifts, and even bipedal mechs, using the enemy’s own firepower against them. While the levels removed the complex branching paths of the previous installment, the abundance of traps, automated mechanisms, and sci-fi laboratory designs maintains the fast-paced, exhilarating feel expected from a 2D run-and-gun shooter.

Market reception was sharply divided. Positive reviews praised the “return to traditional military style,” citing the tight level pacing that recaptured the hardcore battlefield atmosphere of the original, and noting that new characters Trevor and Nadia injected fresh energy into the series. The PS2 port offered stable performance and additional practice modes, winning favor among console players.

Criticism focused on the “recycled pixel assets.” Due to the change in development teams, much of the background art and enemy models were reused from previous entries, leading some longtime fans to feel the game lacked originality. Additionally, compared to the expansive cosmic scale of Metal Slug 3, the levels in this installment are smaller and visually less striking.

Despite these criticisms, Metal Slug 4 remains a solid and exhilarating sequel, successfully preserving SNK’s legacy during a period of corporate transition, allowing fans of 2D shooters to continue enjoying the explosive, over-the-top destruction the series is known for in a new generation.