Thanks for the replies. The car has just over 99K miles on it, but I'm not sure of when the last time the transmission fluid was changed. I'll check it first thing in the morning.
I'm beginning to think it may be an electrical problem and not a trans problem, because it will randomly go on and off for reasons I can't figure out.
Originally Posted by brickrunner
The button on the shifter is overdrive so that isn't going to take care of the problem, in fact it will prevent the car from going into fourth.
Actually, the button is how you select if you want fourth gear or not. It is essentially overdrive, though. When you press it, a light goes on the dash showing that it will not go into fourth. The same button can be pressed to re-engage fourth, so it was my first instinct when the light went on for no reason. It doesn't work, however.