Synchro is likely gone. Can you double clutch it to see if it would stay in gear?

 

Could also be that 4th gear itself is toast. Does it happen on smooth roads and rough roads? I have heard of transmissions popping out when the car hits a pothole due to worn or broken transmission mounts.

Do you get a slight grind when putting it in 4th? That would confirm 4th gear syncro is dead or almost dead.

 

Could also be that 4th gear itself is toast. Does it happen on smooth roads and rough roads? I have heard of transmissions popping out when the car hits a pothole due to worn or broken transmission mounts.

Do you get a slight grind when putting it in 4th? That would confirm 4th gear syncro is dead or almost dead.

Happens on any road doesn’t grind or make noise as soon as I put into 4th and press gas it pops out of gear

 

Inspect the easy ones first.
1. Check fluid level.
2. Inspect for worn shift linkages at the transmission. Cables, end links, cable clips, etc.
3. Check the transmission mount
4. If everything checks out, it's internal. How many miles?
Shift detents, synchro dog teeth worn, etc.

 

It could be a bad 4th gear or synchro.

 

To my knowledge, yes and no. The grind is when the gears aren't aligning with the keyway to engage. Pop out could be a bad pressure plate. But even then, without opening the case it's just a guess.

Op, drain the gear oil to see if there's metal in the fluid. All I know for sure @AircraftMech406 that the synchros are yellow metal. Using the wrong fluid will cause premature failure.