second and fourth use 6th in between them as in: 4-6-2. howeever, third is on the input shaft and locks into 5th or 6th (as in 1-5-3/4-6-2) as third/fourth are combined gears and only third is activated off it it is hard to tell from outside what is wrong. after all, when you say second-fourth you are infact covering exactly half the gearbox namely;2nd, 3rd and 4th.
so from outside....can't say but at least 50% of selection problems are gearlever, gear selector related OUTSIDE the box so you could look at the selector claw for damage. the earlier bikes get burss on the claws and bind.
if the dogs were gone you would miss gears and overrev etc.
regards
Taffy