IIRC, 4th gear is machined directly onto the back end of the input shaft, which is separate from the mainshaft. 3rd and 5th gear sit right next to each other on the mainshaft. 3rd and 5th, as a pair, are held in place by a single snap ring on the mainshaft. If the snap ring broke, maybe it could cause some carnage, as 3rd and 5th gears might slide free-willy forward and backward on the mainshaft. What this may do is allow the 3rd and 5th mainshaft gears to chew into their adjacent countershaft gears. The sliding of 3 & 5 on the mainshaft could also possilbly allow the 4-5 synchro to get "pinched" between 4th and 5th gear when 3rd gear is engaged. 4th and 5th gear would be rotating at different speeds, placing alot of stress on the synchro pinched in between the two. Also, 3rd gear rides directly on the mainshaft, but 5th gear spins on a sleeve. The 5th gear sleeve is held stationary to the mainshaft via a pin, which locates onto the 4-5 synchro hub grooves on the mainshaft. The 5th gear sleeve pin can shear off or get battered, along with the sleeve itself getting out of inside/outside dia. spec, especially if the sleeve were allowed to travel forward, where the shaft decreases in dia., as this would allow the locate pin to be pushed out of the deep portion of the synchro hub teeth, to a shallower portion where the pin could be battered, and the sleeve would thereby be allowed to spin on the mainshaft.