Have you removed the distributor cap and had a look inside it? It could be that the copper post which feeds plug number 4 has corroded or disintegrated, especially if the cap is old. Also look at the copper arm of the rotor arm. The tipof this can get pitted and worn because of the high electrical discharges from this across to the copper posts. My guess is the dizzy cap and rotor arm need replacing.
Hope this helps.