Diskless setup with NFS_V4

Its posible use currentle FreeBSD on NFS_V4 root?

Explain:

pxeboot do NFS_v3 (not NFS_v4) mount and pass fd to kernel.
In this setup kernel can use only configured (established) nfs fh.
This is not allowed to switch version or some options.

When pxeboot use TFTP (not NFS) kernel (in nfs/bootp_subr.c) do DHCP
discover and don't allow (in nfs/nfs_diskless.c:nfs_parse_options)
'nfsv4' option.

nfs/nfs_diskless.c:nfs_setup_diskless also initialy set

nd3->root_args.flags = (NFSMNT_NFSV3 | NFSMNT_WSIZE | NFSMNT_RSIZE | NFSMNT_RESVPORT);

and don't allow 'nfsv4' option.

Where I be wrong?
How I can use diskless setup with R/O root on NFS_V4 share?