I have no experience with this, but had a look at the code. If I understand it right, by default a single server is allowed on a host, so that in your case a server group can only have 4 servers.

However, starting with microversion 2.64 the server group creation API includes an option max_server_per_host. It doesn't look like the openstack client allows you to set this option, unfortunately.

The API description has an example JSON structure that sets max_server_per_host.

Depending upon the max_server_per_host parameter set in the nova, with 1 per host only 4x servers will be scheduled on the 4x host while the rest will complain about the "No Valid Host" since no more host are left to be hosted.