GTA4 has a problem where, if you have 720p and 1080p enabled on your PS3, it will use 720p. To have it run at 1080p, you have to go into your PS3 settings and disable 720p (this may lead to a problem when you try to play a 720p game though).
When I first started testing the TV with GTA IV, it automatically scaled it to 1080p. The TV itself is 1080p, but after I loaded the disc into the system and pressed the play button, the TV was searching for a signal and started with a 1280x1080, then it rescanned and made it into 1280x720. I looked at the game box and it says 1080p at the highest. Why isn't it playing in 1080p? Is there a way to scale it back to 1080p?