The reason you can not currently transer files larger than 4 gigs is because the current file system on the HDD is FAT-32, which has a transfer limit of 4 gigs. You need to change the file system to NTFS.
So, back up any files he has on his usb hard drive to your internal one (temporarily). Then, reformat the partition on the USB HDD that you would like to transfer the files to using windows xp's built in formating program (Start > My Computer. Right-click on the G: drive and then click format). Make sure that you set the file system to NTFS. Next, click format. When it is finished, transfer all of the files you previously backed up onto your internal drive to his USB HDD. You will now be able to transfer any file size you like, even over 4 gigs.