The RFC states otherwise.
https://tools.ietf.org/html/rfc7530#section-1.4
~:# rpcinfo -t vm19180 nfs 4
program 100003 version 4 ready and waiting
And this is a debian VM with an NFSv4 only server. Can the port 2049 reached, at least?
EDIT: though, you are correct. showmount doesn't work with...