1996 rodeo 4 speed auto tran problem

[wrightrl]

My Sister has a 1996 isuzu rodeo the auto trans will work in 1st, 2nd and 3rd gears but when put in drive nothing, . any one have any idea what might cause this. When you put it in drive it will shift through the gears smooth until it goes to 4th then it is just like you put it in neutral. If you rev the engine up it goes into gear when you let up it goes back to neutral. changed the fluid and filter, fluid was brown, no parts in pan. check transmission light flashing, did not make any differance. there are no fault codes showing. It went out while she was driving made no noise just reved up.