Function gta_iv 

pub fn gta_iv() -> Rule

The video memory GTA IV assumes.

GTA IV (Complete Edition). The switch overrides the game’s own video-memory detection, which mis-reads the reported figure and falls back to 512 MB; 2048 unlocks every quality tier while staying inside the 32-bit address space. Nothing else is needed here: the D3D9 quirks this game wants are backend behaviour, so they live in mtld3d’s own built-in profile for GTAIV.exe and apply whichever launcher started it.