Strange – here on a Leap 15.5 client mounting a QNAP NAS box (NFS Server) within a private LAN, NFS v4 is behaving as expected – with IPv6:
NAS-Bureau-001.fritz.box:/NFS on /mnt/NAS-Bureau-001/NFS type nfs4 (rw,relatime,vers=4.2,rsize=131072,wsize=131072,namlen=255,hard,proto=tcp6,timeo=600,retrans=2,sec=sys,clientaddr=2001:xxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx,local_lock=none,addr=2001:xxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx)
But, I’m on a private LAN behind a DSL Router which has a Firewall in addition to, the Firewall at my ISP – therefore no Firewall in use within the private LAN …
Your mileage may vary …