1998 Ford Mustang Engine Misfire

My local service station says I have engine coolant leaking around the manifold or through the intake (on the service report it says the intake is leaking coolant). This was the report before my engine misfire problem began. When I brought the car back in for misfire, they said it needed new wires, and a new number 4 plug and that I needed a redesigned intake manifold. They said coolant was leaking into number 4 and it would continue to corrode and misfire. This was going to be quite expensive (roughly $1600), so I opted for now to just put in the new wire set and new number 4 plug. They noted on the service report that the misfire would come back. What do you make of the problem? Could this remain just a problem for the number 4 plug and be some time before it happens again, and if so, if I did not want to make a major repair at this point, why not just do-it-yourself replace number 4 each time. Or, is this likely a leaking problem that will spread. The service station said this was a common problem with Ford Mustang engines.