.004 is the same as zero when I'm working on my cars.

That piston needs to come out and both it and the bore need inspection. Corrosion on either is no good. I'm a bit of a risk taker and have been known to use steel wool to buff back down the corroded parts. Blasted them real good with brake parts cleaner and then wiped it all up with clean fluid. Never has caused me any grief since. I actually found brake jelly (used to be fluid) behind my pistons. There is NO WAY a flush could have cleaned that stuff out. It sure as hell could have prevented it though.