4th gear is direct drive through to the transfercase. Normally the worst case is you loose all other gears due to them all using the layshaft but 4th should still be available. I lost a layshaft bearing on my LT77 and I got the 90 home using 4th gear combined with low and high range. It seems odd that 4th is the only gear you can't get, I would have to guess a problem with the shift linkage of some sort.