You need to copy all the files from the game you have backed up into the folder where you have installed the .pkg (should be in a folder something like dev_hdd0-game-[or whatever the game code is] e.g BLUS01031 yours will be different for GTA4 and the folder should only be a few MB)

Note don't replace any files that are in the .pkg folder just copy the ones that aren't already there

if you used multiman to backup the game, the game would be located under dev_hdd0-GAMES-

hope this works for you

Wow, never knew about that. Thanks man, you really helped me out. I've been trying to figure this out for a while and I never knew it was that simple.

no worries, took me forever to work out n there doesn't seem to be any good tuts explaining it for the idiots like me...