4th gear keeps poping out when I let off the gas, what's wrong?

Modern cars use synchronizers in order to avoid the need for double-clutching. A synchronizer, or "synchro," lets the collar and gear synchronize their speeds while they're already in contact but before the dog teeth engage. Each manufacturer's synchro is slightly different than the others, but the basic idea is the same. For instance, a cone on one gear will fit into a cone-shaped depression on the collar. The gear and collar synchronize their speeds thanks to the friction between the cone and collar. Then the outer part of the collar moves out of the way so that the gear can be engaged by the dog teeth. When they go bad it can let it pop out of gear.

Or bushings & forks are worn out I think they can be replaced from outside the case, have check the book