Wow, everything seems to be coming in pairs today, I just answered one like this a few hours ago!
About the only thing you can do without pulling the transmission out, is to replace the shift solenoids. Their are two shift solenoids, mounted side by side towards the rear of the valvebody (closest to the crossmember) and they are both the same part number, so I would reccomend replacing both. I have seen in some instances where a bad shift solenoid caused a loss of 4th gear only, so its worth a shot! The solenoids are relatively inexpensive ($15 each, approx)
If that doesn't do it, then you will have to pull the transmission out, as the band may be burnt. The band is used for second gear as well, but the 2nd servo is much larger than the 4th servo, so when the band starts to go out, 4th is the first to go.
Good luck and please let me know if I can be of further assistance!