book your next one without telling anybody, obviously only the instructor as you need the car, right before the test do a quick 30 min drive to ease you into it, try and get an early slot on the weekend
well i had an early slot, it felt a bit like a ghost town which was weirdish but it might help you calm down if not many cars are about, when this is the case they usually take you up dual carriageways, that was my comfort zone
when you have lessons are you pretty much flawless and the instructor barely points out mistakes?
just think what's the worst that can happen by not passing? no one's going to die, you are making the actual test process bigger and more daunting than it actually is, think of it as another driving lesson or mock test but don't attach so much importance on to it
good luck and don't view it as your 4th test, think of the ones prior as practice attempts and view this as a fresh attempt with a new mindset of PASSING