so when your WOT, you going over 120??? and then trying to shift into 4th????
1st gear is 45ish
2nd is 85ish
3rd is 125+
then its 4th gear.
at WOT
which is extremely fast.
I think you need to count the shifts to make sure exactly what gear you are having trouble getting into.
Maybe try a tune up and then see what your MPG's end up being at.