From Linux NFS
Your client should automatically negotiate the highest NFS version supported by both client and server. If you want to turn on 4.1 while it's still experimental:
On the client side: use the latest kernel possible (at least 2.6.31). Mount with:
mount -tnfs4 -ominorversion=1 server:/export/path/
On the server side:
Follow instructions for exporting nfsv4.
Build and install the most recent nfs-utils possible (at least 1.2.0). Restart nfsd.
Check that /proc/fs/nfsd/versions on server says "+2 +3 +4 +4.1". (The "+4.1" is the important part.)
Checking that 4.1 is actually working: watch some traffic in wireshark. The rpc header should list the program version as "4", and the nfs arguments should list the minorversion as "1". Also, all compounds except the first should start with a "SEQUENCE" operation.