mount -t nfs4 fails w/ permission denied; how to debug? (FC3)

[Will Partain]

In a test setup with FC3 client and server, I can successfully

   sudo mount -t nfs albus:/._disc1/sysadm/test1 /mnt/test1

(getting an nfs v3 mount).  If I instead...

   sudo mount -t nfs4 albus:/._disc1/sysadm/test1 /mnt/test1

... I get "mount: permission denied".  Nothing appears in the logs of
either machine (well, one uninteresting SELinux message; SELinux is
not enforced on either machine).  I believe that bits are getting to
the server: certainly running ethereal against port 2049 on the server
shows chit-chat at the expected time.  This is without Kerberos,
i.e. it should be doing sec=unix.

So the question: how can I get something to tell me more about what's
going on?  Thanks,

Will

PS: Ideally, I would like for an FC3+ server to dish out nfsv4 to FC3+
clients, and nfsv3 to less capable ones.  Plausible?