Most common cause is worn clutching teeth on the 4th (input shaft) and the 3-4 slider. I would first check the obvious shifter linkage adjustments but if it goes in solid but pops out as soon as you let of the gas it probably needs a new input and slider
I have a photo to back this up. My transmission (in my 71 Challenger) kept popping out of 4th and I finally got it into my mechanic and this is what he found. I was fortunate in that it didn't damage the trans case but it did require an entire rebuild because of all the metal in the tranny. In my photo, you can see the teeth were wearing against the case when the input shaft was shoved forward. I don't remember all the details as this happened in 1973-74. It cost me almost $200 back then. I now have a great clutch alignment tool though. Get it taken care of as soon as possible. Note on the gear teeth how it was rolling over the edge of the teeth. It is cut back into the gear about 1/4".