How was the fluid level?
4th is a 1:1 ratio. It shouldn't be a problem at all.
Just for kicks, try putting it in reverse with the engine off, then see if you got it in. If it's a synchro problem it might be hanging up and this removes the clutch/pressure plate from the equation.
Fifth and reverse share a synchro assembly. There is an idler for reverse. It shouldn't be affected by driving in 4th though.
Otherwise it sounds like a damaged shift fork.