Hey,
ACS 3 and ACS 4 are windows based servers.
ACS 5 is a Linux form (Centos to be precise).
2 Flavors for the ACS 5:
-Appliance 1120/1121/3415
-VMware ESX VM
ACS 3.x can be upgraded to 4.2.1 max.
ACS 4 to ACS 5 will not be upgrade, it will be migration.
For this you need the new purchase of ACS 5.
The migration process is somewhat helpful but some customers don't prefer that.
You should have a word with TAC of whether it suits your environment or not.
There are lot of things that you need to configure from scratch on ACS 5, so re-think the migration.
Regards
Ed
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Please Rate if helpful.
Regards
Ed