I had issues umounting my NFSv4 shares on an IPv6-only network. When trying to umount the share, it said that it couldn't find the mount in /proc/mounts:

~# umount /mnt/nfs/portage
/mnt/nfs/portage was not found in /proc/mounts

The solution: copy /proc/mounts to /etc/mtab, and the umount works correctly again.