Thanks for your question.
So can you get it into the other gears at all?
Well if you cannot get it into any outher gear but 4th, it sounds like either the shifter rods inside the trans, detents, shift forks etc. It sounds like like from what you're saying the trans will need to be torn down and inspected or maybe try finding another junkyard trans, which might be cheaper. I would also just to know, what the fluid level is inside the trans. The fill plug is on the drivers side. It should be at the bottom of the fill hole. If it is way down, this may be why the trans gave up on you.