Hi Lavanya
Older versions of the CA Single Sign-On agent objects used a security model that featured a shared secret that is stored on the Policy Server and in the WebAgent.conf file. These agents are named 4.x type agents. You can specify support for 4.x agent functions when creating an agent object in the Administrative UI. Later versions use a trusted host object on the Policy Sever instead of the shared secret security model.
You would be using the 4.X and 5.X agent support when creating a custom agent, which you can do using the SSO Development kit. See the programming reference guide for more information.
There are also some products with embedded SSO agents (e.g. IdentityMinder) who will use a 4.x/5.X communications mechanism. Another situation where you would be using them is if trying to use the old FSSUI fo administer SSO.
Thank you