Do you need .net 4 on SBS 2003?
Probably not. And if you install it on that platform be prepared for a new error message - an indication that a service is not running:
SBS 2003 Performance Report indicates an automatic service is not running:
I'd recommend not installing .net 4 on SBS 2003 as I see no need for it. Unless some application demands it, skip it. It's an optional update on this platform and I'd recommend not installing it from the get go. If it's installed, uninstall it.
Do you need .net 4 on SBS 2008?
Probably not. Only worry about patching it if you have an application that installs it. Just because it's offered up as an optional update, doesn't mean that it's needed. If you decide to remove it, you'll find that it doesn't cleanly uninstall. This is actually documented in the .net 4 readme so it's actually an expected known issue.
Do you need .net 4 on SBS 2011?
It actually ships with .net 4 so DO NOT remove it on the SBS 2011 platform. I haven't seen .net 4 have issues with patching, but if you do, don't use the Aaron Stebner .net tool, merely go into programs and features, click on .net 4 and choose the repair option.
So far I've seen doing this fixes up .net 4 to allow for any security updates.