Hmm... for whatever reason my 2nd reply did not even post... I'll try and paraphrase what I typed before. Hopefully it will actually post.
There would be a large difference between 720p and 1080p. The GT 750m is pretty close to the performance of a desktop GTX 550 Ti and the Radeon HD 7750. Both would have about 67% the performance of my desktop Radeon HD 5850.
GTA 4 was not a very good console port. With my HD 5850 at 1920x1200 (just slightly more than 1080p) and medium settings my frame rates are between 22 FPS - 35 FPS from what I can remember. Of course that is with an older C2Q Q9450 CPU from 2008. If I had a more current CPU the performance will probably be higher at around 27 FPS to 44 FPs, but that is only a guess.
Rockstar...