My transmission was replaced by a previous owner and it did not stay in fourth during WOT. I could not use more than half throttle or it would downshift to third regardless of speed. I managed to find instructions on how to fix this problem and this is what I did:

Then I put everything back in the reverse order.

When the stick is in third the tranny is exactly the same as before.

With the stick in fourth it will rev thru second just like before and then shift to third. Shortly after it will shift to fourth and stay there.

To get the best performance I have to keep the stick in third and manually shift to fourth when the RPM's get to high for third.