GTA4 didn't move much hardware in NA.
Based on VGChartz numbers it moved around 200-300k PS3 in Others though. Hard to tell the exact the numbers as there was a small GT5P effect too but if you check the average weekly PS3 numbers in Others for the last 4 weeks compared to what they were 1-2 months ago, there is a noticeable difference...
The difference between Others and NA ?
360 hasn't sold as well in Others as in NA so there was still a pool of hardcore gamers that had not bought a HD console. That and GTA4 was bundled in Others....