Misfire

Do some testing, don't replace anything, yet. Use a digital multimeter. One lead just touches the spark plug wire, you don't take the plug wire loose or penetrate the insulation, just touch. The other lead goes to engine metal/ground.
The meter reads an inductive voltage, sometimes it's around 1/2 volt. As an example, if four plug wire read around 1/2 volt and two read zero, I know where to start looking. You have to be careful suspecting a coil, I've seen faulty ignition modules, that would fire two coils and not the third one.

Just to add, what if the problem isn't spark? Check spark--fuel, use a fuel pressure gage. Then, check compression.