"A recoilless rocket launcher that fires 84mm projectiles. The piercing power of the rocket causes more damage than the explosion."
The ATM-4 can't be found in the main game, it can only be unlocked by completing any of Leon's campaign on Hardcore with an S+ rank or by purchasing the "All In-game Rewards Unlock" DLC. It features infinite ammo by default and is available to both characters in every difficulty once unlocked.
In addition, this rocket launcher is different than the Anti-tank Rocket which is used by Leon and will also not harm the player. However, just like the Anti-tank Rocket, it will terminate the Super Tyrant in one shot -or two in Hardcore- immediately upon use, instead of advancing the fight.
It will also cause the Ivy Zombies to be knocked down and burst into flames when fired upon (although it is described with AP rounds rather than Incindiery), quickly disposing of them. It has a faster rate of fire compared to the Anti Tank rocket launcher albeit slightly smaller blast radius.
As with any other bonus weapons carrying the infinite feature on them, using this weapon penalizes players from achieving S+ rank on a playthrough. I.e. players would still be required to play through Claire's campaign on Hardcore without it to achieve a S+ rank to unlock her counterpart of unlockable weapon.