The torque converter locks up in third gear and in fourth. You are probably feeling the TC locking. That will drop you a couple of 100 RPM's.

On the other comment, I have been Full Throttle in 4th. Just before you hit the speed limiter. Without the limiter, I don't think fourth will be any problem at all...

My name is Richard. I was a Contouraholic. NOW: '02 Mazda B3000 Dual Sport, Black BEFORE: '99 Contour SE Sport Duratec ATX Spruce Green PIAA 510's, Foglight MOD, K&N Drop-in