For some reason NFSv4 has never worked for NFS booting on any of the installs I’ve configured, which was always solved by adding ‘,v3’ to the nfsroot in the cmdline.txt. However I don’t see a way to specify this in the preseed.cfg used by the installer.
I did find a work around, which was to force my NFS to present v3 only.
http://andy.delcambre.com/2007/06/25/disabling-nfsv4-on-ubuntu.html