As far as I know, there are no plans for another PSP GTA game, which is a real shame as the platform could really do with it at the moment. To be honest, unless Sony pulls some sort of miracle out of the bag, I think it's unlikely that the PSP will see another GTA game in its life cycle, but I'd love to be proved wrong.
Even if GTA IV did come to a handheld console however, the limitations of the DS and the PSP would mean that the game would be scaled down in every department, so it wouldn't be like you were getting the same game but on a handheld console. If it's just a new handheld GTA you are after, you would do well to keep an eye on GTA Chinatown Wars for the DS. We haven't heard much about it yet, but you can read about it at the link below: http://www.pocketgamer.co.uk/r/DS/Grand ... asp?c=9033
Hope that helps