Grand Theft Auto IV was never released on PlayStation 2. If you're searching for a PS2 version, you won't find one—and there are clear technical and historical reasons why.
If you want to play on computer, you'll need at minimum: Core 2 Duo processor, 2 GB RAM, and an NVIDIA 8800 GT or equivalent graphics card.
Bottom line: to experience Liberty City in high definition, you'll need a PS3, Xbox 360, or a capable PC. The PS2 era of GTA ended with San Andreas.