On Slackware-current the default kernel supports NFSv4 but the user-space nfs-utils does not. To have a complete NFSv4 support in user-space you need:
You can find all of them here
. Take a look at the bottom of the README
too. Without Kerberos there is no much (if any) difference between V3 and V4. So if you don't need the enhanced security of NFSv4 you can stay with NFSv3.