The MP4 container will probably not accept xvc content officially, it has relatively strict specifications. The
ffmpeg linked above by jonatans may do so due to a high similarity to other MPEG video content formats. (Please note the difference between a
container format and a
content format; .mp4 is a container, .xvc is a raw video content stream.)
It's more likely that MKV may accept xvc content some day, but check their changelogs if a current version does. Version 30.1.0 "Forever And More" (2019-01-05) did not yet announce that in their docs/NEWS.txt of the MKVtoolnixGUI installation.
Disclaimer: Statements above are my humble opinions, I may be wrong.