Have you just started playing GTA 4? GTA 4 was poorly ported to PC when it came to PC and almost everyone had this lagging and fps issue.
BTW, have you patched the game yet? I believe patch 1.4 and/or 1.5 can solve your problem, it did for me.
I just had a question, if the game is soo poorly ported to the PC and eats up resources like crazy why are there these amazing realism mods out there that run like butter. Oh thats right takes a real PC to run them got ya =P