Hello Greg
I did some tests: I install sp5 over sp4 and also install it over a fresh installationof AS and I can tell you that I don’t have any issues.
For the possibility to uninstall a SP can tell you that none of the service packs from AS2010 were build to be uninstall.
SP-5 is safe to be install. Please read the "AS-What is new-2010-SP5-EN" to see what is improving.
Best regards
Valentin Dumitru