rough and knocking engine Help Please!!

One day the car ran fine the next day the engine was making a loud knocking noise and running very rough , making the car shake. When one of the wires to the spark plug is removed the car runs better but then is only using 3 cylinders. (guy told me this) All the spark plugs and wires to the spark plugs were replaced.

What are things to check or replace to fix it?

will it hurt the car to drive on only 3 cylinders?