is transmission shot or sensor

i bought filter from advanced auto parts was wrong says O'rielly auto parts. it fell out and truck stopped pulling. i opened it up and replaced. truck still weak taking off, week later stopped again, quickly shut off and opened back up. filter in pan again, took bushing off filter tapped it in transmission. with socket and replaced filter etc. truck will smoke tires off it but at 40 mph slips out.soon as rpm's go back down it pulls strong again. everyone tells me its fourth gear sensor, please help. thank you, Nate