There are 3 versions of ISPmanager 4: Lite, Pro и Cluster.
ISPmanager 4 Lite version has only the basic features but is enough to manage your own server or VPS. It includes the management of all supported software (WEB-, DNS-, mail server and many other things), unrestricted users and domains quantity support.
ISPmanager 4 Pro version, including Lite features, supports resellers creation, many server unification in single entry point, system limits configuration for users and some other features which permit to provide hosting services.
ISPmanager Cluster version is a new ideal solution with all Pro version features created for cluster hosting providers. It has no single point of failure and any website dependences on the server, so that one website can be processed by many servers in one time!
You can read the details of versions’ difference on the website of ISPsystem: http://ispsystem.com/software/ispmanager/litevspro/
The guide of ISPmanager version change on the server