I'm no expert on aircraft piston engines, but I've rebuilt plenty of car and bike engines over the years.
If you're getting good and near identical compression on all cylinders I doubt it's anything serious.
I would guess it's probably just oil filtering down from the head and entering the cylinder via an open valve.
If you wanted to be sure run the engine again and put the #4 in a position with both valves closed, leave it for a couple of hours and have another look.