SP4 is an unofficial rollup for SP3 + patches. So SP4 is SP4 with no other additional patches after required, unless Microsoft decides to release another patch outside of the support period which I highly doubt.
SP4 = ( SP3 + All Patches to end of support ) Rolled up into a single SP4, which is unofficial and was created by someone who was creative in making their own roll up / service pack.
So a system SP3 patched all the way forward is the same as a system patched to this so called SP4. I doubt that the system will report as being SP4, it will likely state SP3 even after SP4 is run.
