Nothing official yet. We have a page on the site which is a guess at best ( http://www.gta4.tv/page/38/pc-version-specifications/
) but those are probably complete rubbish because:
You need more than 768MB of RAM to run Vista alone, let alone GTA4 on top of it.
I can't see a Pentium 4 being able to process Euphoria on the scale that it appears in GTA4
The ATI 9800 Pro is about 4-5 years old now and, if it did actually work with GTA4, you would barely have any textures at all.
The recommended specs are a bit more realistic but the minimum doesn't seem enough to me.