WD 45 won't stay in 4th

First off, every time you use 4th and it pops out you can only make it worse. You could pull the shifter and see if the bottom end is wore out, but more than likely it is the shift collar and the splines on the gear are rounded off too much. If you have slop fore and aft in the trans gears you will need to address that. I'll bet you have to disassemble the rear end to fix it. Drain the tranny and rear end, remove back wheels and final drives, pull back cover of, remove shift tower, remove ring gear and pull the pinion shaft out catching all gears and collars as it comes apart. If you freeze the pinion shaft before reassembly, and you work fast, you can get the tranny back together fast enough to push the pinion into the rear bearing.
Good luck,a repair manual will almost be a necessity.