I tested by uh... building a "mirrorfs" and compiling this project in it :-) (cargo does stress it pretty hard). I have not looked for POSIX compliance testers. Is there one you can point me to?

NFSv4 did throw out a lot of the legacy stuff, but also added a bunch of other state information like locks and delegation which is quite a bit more annoying to implement. Technically cos this is meant for "localhost-mounting-only", delegation should be easy to build (since we are expecting only exactly 1 mount). But v3 had far fewer APIs to implement, and so is a good starting point.