Transmission stopped shifting into third or fourth gears, what could have caused this to happen?

This means something inside the transmission has broken or the shifter has worn out. Here are some diagrams to look at. As you can see there are several bushings that can wear out and not allow the shifter to fully engage the transmission gears. I would use a flashlight and have a helper shift the transmission through the gears (engine not running) to see if you can see something loose or not moving. Check out the diagrams (below). Let us know what happens and please upload pictures or videos of the problem.