We use our Wii to play MP4s from our harddrive, over our wireless router. I have M4V files. I can change the extension to .MP4 and they are playable. Is there any long term reason why I should convert them instead? I did notice that the M4V "container" makes a larger file. Maybe because of the DRM within M4V.
If you come across a player that doesn't want to play them, I think you can have YAMB remux them to .mp4 without re-encoding. http://www.videohelp.com/software/YAMB
YAMB looks like a potential nightmare waiting to happen.. read the comments below the download before going any further..