Modders have created "Total Conversion" mods for GTA: San Andreas that replace the map, characters, and HUD to look like GTA 4. These are often compressed heavily to fit on a PS2 disc. You will see:
If you want a GTA game set in Liberty City (the same city as GTA 4) on your PS2, play Grand Theft Auto: Liberty City Stories . Gta4 Ps2 Iso Highly Compressed
The reason a true GTA 4 ISO for PS2 cannot exist has nothing to do with compression and everything to do with architecture. Modders have created "Total Conversion" mods for GTA:
Rockstar Games did actually release a version of GTA 4 for the PS2—sort of. They released Grand Theft Auto: Episodes from Liberty City (containing The Lost and Damned and The Ballad of Gay Tony) on the PS2. However, these were strictly top-down, retro-styled titles that looked nothing like their Xbox 360 counterparts. They were fun, but they weren't the "highly compressed" 3D open world players were searching for. Emulation/burning considerations:
The PS2 had 32MB of RAM. The PS3 had 256MB (plus 256MB of video RAM). You cannot compress a waterfall into a coffee cup. The complex AI, the real-time lighting, and the vehicle deformation that defined GTA 4 were hardware-dependent features. No amount of WinRAR wizardry can make a PS2 render the reflection of a neon sign in a rainy puddle in real-time.