Why is my transmission not switching to 4th gear?

What is your question?

I have put A crate engine into this vehicle and now the transmission will not switch to 4th gear. When driving 55-60 my Rpm's are around 2500-3000. When I left off the gas it acts like it upshifts. But when I apply the gas again it goes right back to high Rpm's.I have read up on this issue and one common fix was the throttle body. The transmission was fine before engine replacement. I put A long block in so the only used parts put back on it was the Intake/Throttle Body and the alternator. Please let me know what could be the problem before I start sinking money into parts. Thanks