I started having trouble getting into 4 th gear, then it started popping out. It popped out of 4 th and i put back now it will not come out of gear even with the engine off. Could it be air in slave cylinder (i filled the day before, it was low but not empty. Or could it be something else ??

 

If the transmission pops out of gear, it is most likely caused by bearing failure internally. The ones that pop are 'pushed' out of gear by the counter shaft being at an abnormal angle with the input shaft. The resulting force pushes the fork(I guess) against the collar, and there you go. OR, the input shaft meets the output shaft via a needle bearing, with the other ends of the two supported at the ends of the transmission by ball bearings. If these wear out or are 'cooked', the shafts will not meet straight, and will put force on the collar.
To fix, you have to remove, take it apart, and replace the worn stuff. You may have damaged parts by now.
One other possibility is that the linkage is so sloppy that it does not engage the collar properly to lock the input to the output shaft. 4th gear is 'straight through', input locked to output. OD uses a gear at the end of the countershaft, to transfer power to a gear on the output shaft, with the countershaft gear 'bigger' than the output shaft gear, giving overdrive.
tom