well, stuck in 4th isn't a good sign. If your shifter bushings are intact, and it's locked in gear its internal. It's a possibility a roll pin in the top cover slid in the wrong place, preventing you from leaving the 4th position.. 4th gear isn't actually a "gear", it's a collar that locks the input/tail shaft together. In other words 4th gear is actually a "direct drive". If above mentioned isn't it, you have a catastrophic failure somewhere. How's the fluid level, and color? After checking everything you can externally, fluid (to determine level, color,or any chunks), shifter bushings, you need to get the top cover off the trans and have a look see.
ati190