How to get nfsd to serve NFSv4 on High Sierra?
The version of nfsd on the Mac does support v4. For ACL support you definitely need to connect using v4.

Your failure to get a v4 connection is probably on the client side. Most NFS clients default to trying v3 first then falling back to v2 if unsuccessful.

On your client side you should specify v4 using the -o vers=4 option to your mount command. Do a man mount_nfs on the client to check that it supports that syntax rather than the old deprecated -o nfsv4 syntax. You could also use -4 but that is also deprecated and highly discouraged.

To get ACL support in your client you may have to add the -o acl option as most clients default to no ACL support.

A close reading of man NFS and man mount_nfs on the client is highly recommended.