While it's certainly best to have a working 4th gear "lockout" switch (which, of course, locks out the od), there are ways to deal with it temporarily. I was faced with a non-functioning switch on a car I purchased years ago and not having the time to deal with replacing it immediately, I wired up a key buzzer and warning light using one of Volvos simple 3 prong relays. I took + to the extra relay from the OD relay's far side and ground from the reverse light relay. Therefore if I inadvertantly left the car with the od switched on, the buzzer and lamp would activate as soon as I went to engage reverse. This method also allowed me to play with the od in gears other than 4th, just as MGs (3rd & 4th) and Triumphs (2nd, 3rd & 4th) are wired. It was interesting having a 7 speed for a while.