NFS4 isn't working correctly - there is a large hang mounting or unmounting - if you turn on debugging ;
echo 1 > /proc/sys/sunrpc/rpc_debug
echo 1 > /proc/sys/sunrpc/nfs_debug
you can see what looks like a problem with RPC, but I cant work out what the issue is.
I can mount from other live systems ok to my server (have tried also on several different server boxes) and same issue, although all running Ubuntu.
nfs3 etc are all ok, just nfs4 is the hang.