"Cannot restore featureCompatibilityVersion document. A 3.4 binary must be used."
"There is no feature compatibility document. A 3.4 binary must be used."

These two messages should give more instruction on how to fix the problem.

"Downgrading files to FCV 3.4"
This message should be more specific about what is happening (or we should remove it from log level 0). The "files" in this case are the "WiredTiger database files", and the "FCV 3.4" is really referring to the "WiredTiger binary format compatible with MongoDB 3.4", and does not actually touch admin.system.version and the FCV value contained within it.
I think it would be best to remove this message entirely.