Where is nudity in gta4?

There is no complete nudity on the main GTA IV story, but the girls at the lap dancing clubs do wear very little in the way of clothing. There is probably some kind of mod for the game which may show nudity, which the dedicated games sites would have knowledge of if it existed. Perhaps try this site and see if there is mention of it, if you want to go down that route:

www.gtaforums.com