The only way to get the rollup is to contact support. It is not published or available thru SIM. They are released almost monthly, however V3 was released at the very end of March and V4 will be released the 1st week of May so April was skipped. I suppose it depends on timing.
Symantec will tell you only to install the rollup if you are experiencing an issue that is fixed with the rollup per the release notes.
I will tell you that I install each rollup after testing it in my lab as I have come across things I've experienced as part of SP2 or a previous applied rollup that may not be specifically addressed in the release notes but is known to fix a current issue. I am currently on SP2V2 as it included platform & agent fixes I requried.
For example. I have an open ticket with Dev & Engineering regarding the automatic agent upgrade for 64 bit resources (Windows 7 in my case). On some machines the agent will fail to upgrade and will end up without any agent installed. What occurs is a related symantec agent servie is getting restarted during the upgrade process and causes the upgrade to fail. It does not happen all the time and I've supplied 7 FULL procmon traces of failed upgrade attempts during the upgrade process.
I have been told that the code in the V3 release is much more "solid" code for the agent and that it performs a bit better on resource usage and may resolve my upgrade issue. I also know there is a code change for the V4 agent that also addresses a second open ticket I have for a known issue with the SMFDiscovery process on 64 bit OS's. Looking thru the preliminary rel notes for V4 I don't see these 2 items listed but have been told the updated agent should fix them.
I am skipping V3 and waiting for V4 because of the on-going rollout of the SP2V2 upgrade we're still performing (I did not want to introduce a new agent mid-upgrade process).
This is why I apply the rollup, because issues not listed specifically in the release notes may be fixed by the patch.
Also, each rollup is cumulative so you only need to apply the latest version and you recieve all fixes from all previous versions.