In the real world only a small minority even use NFSv4 and that mostly for the figleaf of encryption via (yuck!) Kerberos. As a (too) long time storage industry person, I have hard numbers on this.
AWS's EFS is mounted with NFSv4 on Linux instances. Had to fight against race conditions between clients recently (per client directory caches,) but that's a feature of NFS, not specifically v4.
