I've started ripping some DVDs using Handbrake for Windows, ATV preset. I notice that when I pick a destination file for the output, it wants to default to the .mp4 format. When I try to play back the encoded file, Quicktime gives me an error. If I rename the file and change the extension to .m4v, as some of the other files are, then it warns me that changing it could cause problems, but the file then plays fine in quicktime/iTunes. What gives? What's the difference between and MP4 and an m4v? Thanks.