4ac missfiring help!

[pro-16t]

As i started the car one morning it seemed be running on three cylinders but not that bad, once the car warmed up the missfiring went away, as time progressed the problem got worse missfiring even when the car was hot. So i took it to a mate to have a look at it he suggested that one of the valves most likely no.4 wasn't seating propley i also noticed that when i took lead no.4 off the missfiring really wasn't getting any worse and that the car was drinking alot of water further more when car was almost bone dry the missfiring dissapeard so anyway i changed the dizzy cap, plugs, leads, rotor, still no change. So i thought time to rip the head off i did and took it to get reco'd when i told the reco bloke what the 4ac had been doing he said that the water housing on the back of the head was corroded and after 20yrs the back off the head had been eaten away thus water leaking in around the no.4 cylinder making the 4ac missfire, another bloke at the reco 's also said that the head gasket was stuffed thus water getting in etc: so i took there word for it. I put the 4ac together just before new years started her up and she was like new until couple of days ago when this f#####g missfiring has started again and i can't for the life of me figure it out someone help!

[pro-16t]

come on people any suggestions