FOUR COATS ??? This should have been done in only ONE Finish Coat! This tells me your problem is that you are using an inferior grade of Paint (a very common problem these days)… even tho you may have paid top $$$ for whatever paint you used!
So for the side that you have left to do… sand it down with coarse paper using a block sander… then Prime it with Pigmented Shellac Sealer Primer… then Finish coat (one coat, use a 1/2 in nap roller and double roll with a "dry roller" -see my site for help-) but use a HIGH GRADE PRODUCT like Sherwin Williams A-100 (use semi gloss)… Good Luck!