Its 1 Domain/ZONE pro plan.
So if you do have a setup like this:
www.domain.com (Site 1)
sub2.domain.com (Site 2)
sub3.domain.com (Site 3)
sub4.domain.com (Site 4)
You would be able to make all 4 sites/installations profit from your one Pro-Plan.
But if your setup is like:
www.domain1.com (Site 1)
domain2.com (Site 2)
www.domain1.org (Site 3)
www.domain2.org (Site 4)
You would need 4 Pro-Plans to make all 4 sites (different Domains) profit from the Pro functions
SHORT: its paid on Domain base