Set Default NFS v4 when connecting from Mac OS X Lion
Hmmm, seems we need to have this question re-answered every major release of OS X.

So I can only say this works on 10.10.x :)

From OS X's man page for mount_nfs

"nfs.conf(5) can be used to configure some NFS client options. In particular, nfs.client.mount.options can be used to specify default mount options"

To get the Finder to default to using NFS v4 add the following to /etc/nfs.conf using the terminal command "sudo vi /etc/nfs.conf"

 nfs.client.mount.options = vers=4


Again, to check after mounting use "nfsstat -m" and check the "NFS parameters".