Transmission shifting problem?

My mechanic has had my car for over 1 month.
The car only shifts 3 times when it is suppose to shift a 4th time.
It gets up to around 40 miles per hour and it should shif into the last gear but it doesnt.
The mechanic that has my car is very honest but he cant figure the problem out.
He said when he takes off the negative terminal the car shifts all 4 gears but only once.
Doesnt make sense to me?
He tryed to put a speed sensor on the car but rthat didnt work and he returned the part.
He seems to think it is nothing on the inside.
He also says he has to bring the car somewere else with the proper cmputer to check it out.
I am not sure what to do at this point the car runs fine but you cant go past 40 or it just winds out and doesnt shift the last time we are afraid to make the tran worst.