I think the 4000 over the 500 will be smoother and handle more oil and be easier to replicate.

As you probably realize by now, polish over any base grit level involves so many variables on the human side of the equation from amount of polish, pressure and time on the spinner, that will affect the final result, it's hard for many people to replicate what they did the first time with succeeding efforts.

Add to those factors, the type of polish, from non-abrasive types, like Snake Oil to extremely abrasive ones like Rough Buff and Storm's Step polishes and you add to the complexity.

Then the polished ball has the potential to be more inconsistent in its reaction than the sanded version. BUT, of course 500 grit + polish will be less inconsistent than 2000 or 4000 grit + polish. Everything is relative.

20 microns is around P900 - P1000 grit range, maybe as low as P800 grit (all FEPA grits). Fairly rough and early rolling. 8/9 microns is in the P2000 grit range.

Hooking or rolling? If it's getting into a roll too fast, then yes, polish will help. If it's too sharp off of the breakpoint, then a 4000 pad might smooth out the motion.