Just an FYI, while I am aware this documentation refers to vCenter Essentials (limited to managing 3 physical hosts), it is likely this would also apply to vCenter Foundation (but with 4 physical hosts):

https://www.vmware.com/content/dam/digitalmarketing/vmware/en/pdf/products/vsan/vmware-vsan-licensin...

"• The vSAN witness appliance must be added to vCenter prior
to the third physical host being added to vCenter.
• A warning message will be generated when attempting to add
the vSAN witness appliance as a vSAN witness host after the three
physical hosts have been added.
Note: The order of operation is important; vCenter does not recognize the
vSAN witness appliance as a virtual host until after it is added to vCenter"