Happened to my 2010 that i just bought a month ago. tThe dealership had a "service bulletin" for the problem so it didn't need to be replicated. It was popping out like once a day at least. They took apart the transmission, threw away first gear, rebuilt a new first gear, rebuilt all the gear synchronizations, and then rebuilt the transmission. Luckily it was under warranty.
EDIT: this was happening in 1st from start not 4th
Its a Jeep thing...get used to it or get the auto! i've had manuals for over 30 years and when I test the Jeep manual, it just didn'tseem right, so I drove the auto and ordered that. If you can get past the winter, the shaking, pog, and long throws and spongy clutch pedal, then you will enjoy your Jeep, otherwise, trade it in for the auto...its a Jeep, not a sports car! Snik snik...
Mine had the "pops out of 4th gear" problem and it got worse as temps dropped.
The first time I took it in, the dealer "Inspected, Replaced Shifter Detent and Shifter Boot per TSB" and that bought me about a week before the problem returned.
The second time I took it in they ended up keeping it and giving me a loaner while they replaced the "Counter shift assembly" which involved removal of the transmission and had to be done by a mechanic Certified for the type of Transmission work it involved. My dealership only has 1 such guy so my guess is it's a rather specialized certification.
When I got the Jeep back I could notice an immediate change in the way 4th gear felt. Before the work, 4th gear was very loose and rattly sounding when at certain RPMs. 6th also felt and sounded the same way. After they replaced the counter shift assembly, 4th gear feels nice and solid and has no more rattle to it at any RPM. 6th gear improved as well. Before the repair, 4th gear never really felt like it went very far into gear. When it would pop out and I'd clutch again to stop the grinding it never really felt like it went in any further. Now, it is very obvious that I'm shifting into 4th. The counter shaft assembly must have taken out all the slop that was in the factory stuff.
I believe this last repair has done the trick. It's been almost 2 months now and no more grinding or popping out of gear.