Probably best not to drive it.
If it's a four speed, it's likely an AT-545 or an MT643.
If reverse is out and drive is okay, it's most likely one of three things.
1. The fourth clutch has failed.
2. Check the selector linkage. It's a simple cable connecting to a lever at the transmission. Make sure when the selector says reverse, the detent at the trans is fully in reverse. It may be out of adjustment.
3. It's both. The selector was out of adjustment failing the fourth clutch.