Is it a 5 or 6 speed? I would get the car up on stands, remove the gray plastic cover over the shift linkage, on the underside of th trans(cotter pins hold them on, see the service manual here). You can remove the two shift cables and try to move the shift levers by hand. If it will not move into fourth with the cables disconnected, I am sorry to say, but the trans will need to be removed and rebuilt.
Good Luck,
LarryB