Hyndai Alantra 2002 Failure

I have a 2002 Elantra with a check Engine, apply the scanner and shoot a dismisssed foal fault cylinder 3 and then in 4, first change spark plugs, then plug cabble, then the ignition coil, subsequently clean injectors, and change the O2 sensor, and the problem continues, the car is normal but the engine has a small failure that does not stabilize it. revolution, what can it be?

You check the compression in the cylinders, you changed two or four spark plugs and just like the cables it can also be some disconnected air mangued check and let me know.-