Is it possible to boot Debian Wheezy from NFS v4?

Bootwing with NFS v3 works just fine. NFS v4 not.

This is in my /etc/exports:

/nfs 192.168.100.0/24(ro,sync,insecure,no_root_squash,no_subtree_check,fsid=0)
/nfs/root 192.168.100.0/24(ro,nohide,sync,insecure,no_root_squash,no_subtree_check)

/nfs/root/www contains the root of the webserver.

The commandline is:

rootfstype=nfs4 root=/dev/nfs4 nfsroot=192.168.100.1:/root/www

fails with mount call failed - server replied: Permission denied.

Mounting from the busybox in the initrd fails:

mount -t nfs4 192.168.100.1:/nfs/root/www /root
mounting .. failed: Invalid argument

Do I need to modify the initrd?