Well, the cabbie was not a very nice man, as John has pointed out to us, so I perfectly believe, that he might have cheated.
Sherlock was right, you can manipulate as much as you want, it's still chance. And if I had been the victim of the cabbie - even, if I would believe in his manipulative super powers - there are ways to bring it back to a 50% chance: I would just have not played the manipulation game of guessing and second guessing with him. I would have said something like 'one, two - out are you' in my head to create a 50/50 chance for myself. The cabbie could not have prevented it. So he was either 4 times lucky, which is not really that outrageously unbelievable (the sample here is not really big enough for viable statistics), or he cheated. If he cheated, John's intervention was literally vital for Sherlock, and I guess that was the main point of that scenario. The cabbie goaded Sherlock into almost doing something unbelievably stupid, and John rightfully calls Sherlock an idiot for engaging in such risky behaviour.