Transmission.

My transmission wont shift on the 4th gear.
It goes to 45 and wont shift no more.
The car runs beautiful except it wont shif the last gear and it just winding out at 45 miles per hour
The mechanic put a speed sensor on the car but that didnt work.
He says when you take the negative terminal off the car shifts all 4 times but only once makes no sense?
He is a mechanic from years back and he has had my car for over 1 month.
He says he doesnt have the proper computer to properly check it out.
As soon as you get up to 40 to 45 miles per hour when it should shift to the last gear it doesnt.
The mechanis seems to think it is not internal but something else but what?Someone said maybe it is a module if I speeled it correct?