I just tried adding nfsv4_server_enable="YES" to my rc.conf and found that after I rebooted the server, my FreeBSD 8 client (still using NFSv3) couldn't connect because there was no RPC mapping for nfs. Removing the option above and rebooting the server makes it work again. Server is GENERIC 9-CURRENT r211027 Client is GENERIC 8-STABLE r211321