And now I completely disagree with L36NotPro. Sorry dude.. trans fluid gets dirty, burnt, and the oil/detergents break down over time.

OP, I wasn't saying that what you were doing was bad. I got 180K and I still go in and out of 4th at WOT. Kinda waiting for it to strip out. And yes, the more info the better. What I was saying is..your issue might be the 4th splines wearing out and stripping slightly as your trans tries to go in and out of 4th. Another possiblility is that the 4th clutches are worn. There's only two actual frictions in the 4th pack.