Best bet would be to put an NTFS partitioned sd card on it, and put your big files on that. I don't know if the phone can read an NTFS partition, though. But it's worth a shot.
Typical 2-hour movie fitted for Vibrant (800x480 30fps) with 2Mbps stream for best quality should be not larger than 2GB. Not sure what would I need 4GB files compatibility for?
For movies encoded in 720p format (1280x720 res), give me one good reason to store such movies on Vibrant. Cheers, Steve