First of all
MP4 (actually "MPEG-4 Part 14") is a
container format. It describes how data is arranged in a file, but is does
not describe how the audio and video is compressed. Though, MP4 files may contain a variety of different
audio formats (such as MP2, MP3, AAC, AC3) and a variety of different
video formats (such as MPEG-2, MPEG-4 ASP, H.264/AVC and H.265/HEVC).
Now, MP4 files may be audio-only (contain only an audio stream), they may be video-only (contain only a video stream) or they may contain audio
plus video. In all of theses cases, it's still simply a MP4 file. And using the
.mp4 file extension would be correct in all these cases. Anyway, since file managers (like Windows Explorer) usually look at the file extension to decided which application they open a file with, and since people may which to open "audio" MP4 files with a different application than "video" MP4 files, this has lead to the practice of using the
.m4a and
.m4v extensions for audio and video MP4 files, respectively. Technically, these
.m4a and
.m4v are still in the MP4 format. There is
no M4A or M4V "format".
Having said that, Apple uses a DRM technology called, euphemistically, "FairPlay". So MP4 files (regardless of what file extension they have) you download from iTunes are probably "protected" by FairPlay. It means that, outside of the Apple ecosystem, you probably cannot do anything with these files. So this is
not about "converting" M4V files to MP4, but about "circumventing" the FairPlay encryption. And there's
no "official" way to do that. That's the whole point about DRM...