Welcome to the forum, lot of knowledgeable people that are willing to help in a clean environment.
I believe he is having a problem with it POPPING out of 4 lo not having problem GETTING it out of 4 lo. Not familiar with the inards of a 300 but if I were looking I woudl look for the syncro/engagement hub and the gear that it locks onto. I would also check the adjustments if any of the shifter, even if it out of adjustment chances are it has damaged something internally at this point. It is my understanding that they are very durable units.
Careful your character's showing