The Boss BV9976B supports playback of JPEG/MP4 and DIVX formats. Double-check to make sure your movies are in one of these formats. Then triple-check. Download a converter to convert your movies to the proper format.
Boss BV9976B manual
It's possible that the Boss unit works with iPod and iPhone 3 and not the four. Does the unit state that it works with iPod/iPhone? Also, you may require a software upgrade