I wonder why you think it is running on only three? But if you're right then the most likely cause is spark plug/ignition system. If you take all the plugs out (with the engine turned off) you should find the end of the plugs dry and a light grey colour if they are running properly. If one plug is wet/darker than the others then that is the faulty cylinder. You will then need to see if it is the plug that is faulty or the ignition lead. In my experience spark plugs will do 20k miles but they won't go on for ever, the gap erodes, gets bigger and then the spark struggles to jump. My guess is you simply need a new set of plugs.
Richard, December 2008
