4160e
You say from second to third it revs up until you let the throttle off but there is no slipping in 1st, 2nd, or 3rd. That sure sounds like slipping in third. The most likely cause is a failed or failing 3-4 clutch. It doesn't ever hurt to scan for trouble codes although I've never seen an electronic issue cause slipping in third.
recap. 1st to 2nd fine. In 2nd it reaches shift point an free revs until I let off the throttle( in 2nd )and it will hit third. Never 4th. I checked 2/4 servo and there is 1/4" inch of depression and feels pretty loose.

