my 05 dodge neon wnt shift into fourth gear. all other gears work except fourth. why could this be and how much is it to repair

I once had that problem and thought it was the transmission and I took it to diagnose, it turned out to be that the battery where the battery seats was so rusty that the battery fell and was pressing on the shifter, once release it was just fine. i had a new seat made for the battery and took care of the problem. check this out.