Sounds like the lock-up to me. Same thing with my 32 gts-t. 1st, 2nd, 3rd, overdrive, Lock-up. You'll note the lock-up only moves the revs down slightly, also once it is engaged it drives like a manual so when you load it up the revs don't immediately change. You can tell the 4th change is overdrive by turning it off, that will also stop the lock-up.