I gave this a small test with an MP4 file on Windows and trying different known MP4 atoms that could hold copyright information. The cprt atom Mp3Tag is writing is not read by Explorer. There is also an alternative ©cpy atom supported by some tools, but that's also not read by Explorer either. The iTunes Metadata Specifications also mention an atom ©cak for "Copyright
Acknowledgement(s)" (whatever this exactly means), but this again is also not read by Explorer.
Out of fun I also tried it with ----:com.apple.iTunes:Copyright (which is the most widely used structure for free text tags in MP4), with different casing of "Copyright", but as expected without luck.
If Microsoft supports copyright information in MP4 it is nothing obvious.
This is also an interesting thread for MP4 tag support in general: