According to tech specs of Apple's products and your own experience, MP4 should be supported by QuickTime, iTunes, iPhone, iPod, and iPad.
So why people are searching for a solution to convert MP4 to M4V? In fact, MP4 is a video container in which video can be encoded in different codecs like H.264 and MPEG-4.
If your MP4 are encoded using an incompatible codec, they are not supported by QuickTime or your device.
Plus, there are limits for each codec if you want to play MP4 video on your device.
If your MP4 files are compressed in other codecs, or the video parameters exceed the limits, you have to convert MP4 to M4V to import to QuickTime or play on your portable devices.
Fortunately, you don't have to be an expert to convert MP4 to M4V.
Just get iSkysoft iMedia Converter Deluxe, and you can turn incompatible MP4 to M4V format in a few clicks.
If you know well about the video parameters, you can make changes to them if you want. Or simply use the preset profiles for iPhone, iPod, iPad, iMovie, Final Cut Pro, etc.
No manual settings are required.