MAC OS X 10.6 (Snow Leopard), 10.7 (Lion),10.8 (Mountain Lion),10.9 (Maverick), 10.10 (Yosemite)
The Activation Server 4 is either hosted or can be installed on an own server. It is a .NET service and needs a Windows server system.
The detail requirements are listed in the Activation Server 4 documentation.