How do I play FLV and MP4 files on my jailbroken iPhone 4, without converting the files?
VLC (google it) plays just about everything. Unfortunately, due to a strict interpretation of some open source licenses (that is, not Apple's fault), the iOS version was pulled from the App Store. 

Fortunately, though, you can still install it by using Cydia. Or at least that's what I've read (I haven't done it myself), see the instructions for details.

Getting the video files onto your phone will be more of a hassle, but I think this might be your best bet. Can't hurt to try.