Here are some benchmarks for the GT 720m.
http
/www.notebookcheck.net/NVIDIA-GeForce-GT-720M.90247.0.html
The GT 720m is ranked between the GT 540m and GT 550m, but closer to the GT 540m. I have a Lenovo Y470 (1366x768; i5-2410m) with a GT 550m and I remember getting pretty decent performance with "medium" settings. Even though the GT 720m is weaker, I think you should be able to play on medium settings. The problem is GTA 4 is a pretty bad console port.
Word of warning. GTA 4 is very CPU intensive. Playing games on my Lenovo Y470 usually causes the CPU to hit 92C. Pretty high, but unfortunately, that is considered "normal" for a CPU in a 14" laptop when playing games. GTA 4 makes my CPU (i5-2410m) hit 99C after about 1.5 hours of gameplay and it started to throttle down which causes stuttering.
Use can use a program called CoreTemp to monitor CPU temps.
The only way to lower the CPU temp in my laptop is force the CPU to run at slower speed. While the i5-2410 has a standard speed of 2.3GHz and can turbo boost up to 2.9GHz (one core, about 2.7GHz using both cores), I basically force the CPU to run at 1.8GHz when playing games, that keeps the temps down to the low 80C's.
That is done by going into the Advanced Power Options, expanding the Processor Power Management section and then changing the Maximum Processor State.
100% = Turbo Boost is on. Core i3 does not have Turbo Boost. 2.9GHz (one core) / 2.7GHz (two cores) for the i5-2410m
99% = Turbo Boost off. The CPU simply runs at stock speed. 2.3GHz for the i5-2410m.
98% = 1.8GHz for the i5-2410m. Your actual clockspeed will likely be different unless the laptop you are buying also has an i5-2410m (doubtful).