According to Duncan Blog under Adding a fifth (virtual) ESXi host to vCenter Foundation - Yellow Bricks the given answer isnt right. There was a bug in previous vCenter versions that the witness ESXI VM counts as a real ESX within vCenter. A workaround was to add this VM first and than your real ESXi Hosts. This bug was fix with u2. Check VMware vCenter Server 6.5 Update 2 Release Notes (scroll down... its the last point).
If you planung a stretched 4 node Cluster you may ask your VMware Sales first.
Regards,
Joerg