Sorry to be so late to the party, but this one has taken me all day (thanks to the excessive speed of Windows updates ?)
My backup drive held Windows 10 version 2004 (19041.330), and it handled MP4 files nicely.
I spent all afternoon and much of the evening updating Windows 10 (like watching paint dry!!!), and a few minutes ago I finally arrived at version 20H2 (19042.844). I don't know why Windows updates wait for ages after downloading before they start installing, or why they take so long to install.
Anyway, NXT4 is still behaving, and handling MP4 files just as it was before. I'd show you another screen clip, but it's the same as the first, so there's not much point. So it seems it's NOT Windows 10 or its updates per se, that's stopping yours from handling MP4s.
Looking for CODEC lists, I found a site which said
"Codec Information Through Windows
Go to Help -> About Windows Media Player, then click on the Technical Support Information link at the bottom. This will open a webpage that lists several bits of information including installed audio and video codecs."
You have to run Windows Media Player (eeew!) to do this, but I did, and got this result for my machine:
If it's any help to you, these appear to be the video CODECs I have installed, and my NXT4 is processing MP4 files properly.