There are several difference between M4V and MOV. In many cases, you need to convert M4V to MOV.
All know that, .m4v is .mp4 files with the WRONG extension introduced by Apple for video, audio. MP4 is a container format. As any other container format like AVI (.avi), Real Media (.rm, .rmvb), QuickTime MOV, MPEG, Matroska MKV / MKA, OGM it allows you to combine different multimedia streams (mostly audio and video) into one single file.
QuickTime is a multimedia development, storage, and playback technology from Apple. QuickTime files combine sound, text, animation, and video in a single file. Using a QuickTime player that either comes with a Web browser or can be downloaded from Apple or the browser company, you can view and control brief multimedia sequences.
There are many cases that you need to convert M4V to MOV, the most typical example is for that, having Apple QuickTime installed is sufficient. Occasionally, a file is not compatible with QuickTime. First you have to convert M4V to MOV.