[John doesn't say anything, but the look he gives Moriarty says 'bite me.' He turns his attention to the smears of blood across the inside of the carriage, looking for any numbers that have been drawn out and eyeing the seats to see if there's any sort of pattern he can see in which ones are covered with blood and which ones aren't.]

I could just go through all the combinations I can think of. Four digits? Probably could get through all the potential combinations.