Three weeks after the events of Metal Warrior 3, metal guitarists Joan (Phantasm) and Ian receive a call from another musician, inviting them to a meeting in a deserted farm. Things soon take a strange and ominous turn, and Ian finds himself on another adventure. His past experiences in the metal scene, and in his struggles against military conspiracy, are about to take on a new meaning as he is initiated into a mysterious order, the Agents of Metal.

MW4 follows the previously established action/adventure formula, following Ian in his transformation into Agent form, with some new twists and features. There are now three difficulty levels, which determine the character's resilience and his carrying capacity (in kg), new special moves for the action/fighting sequences, armor classes and upgrades, more interactive environments, a weapon reload mechanism, a stealth element, which rewards sneaking and non-lethal action, and save slots instead of a password system.