Do you have one of these, link below, that you can point at the exhaust manifolds and try to pin point better which cylinder(s) are missing? That'll help you determine which plug could be bad. Shoot, a friendly mouse could have made a nest in there and be blocking intake passages not allowing gas and air to get to the cylinder. Those stupid things can get a lot of places a person wouldn't think they can.

http://www.amazon.com/HDE-Non-Contac.../dp/B002YE3FS4