Automatic trans eclipse wont shift into 4th gear?

i just bought a 1997 eclipse. on my way home i noticed that it wasnt shifting into its 4th gear. 1st, 2nd, and 3rd work just fine, but it has never engaged to 4th. i tried pushing the overdrive on but it didnt seem to do anything, the o/d light doesnt even turn on. any ideas to what might be wrong with it?