First things first, do a compression test on all 4 cylinders. If they all have compression check the spark of all plugs while you have them out.
You can also determine the exact cylinder by removing the spark plug boot with the car running, if you can tell you removed it then that cylinder should be fine. If no change in idle when you remove one, start with that cylinder.
If one does not have compression do what is called a "wet test" this involves pouring a little bit of oil in the cylinder hole (spark plug hole)and running the compression test again, if you have compression now, bad rings can be a cause;if still no compression you should be able to remove the coupler from the compression tester and hook it to an air compressor with about 25-30 lbs PSI to the line thread the line into the spark plug hole and put air threw it. If you hear air coming from intake, it's an intake valve, if you hear it coming from the exhaust, it's an exhaust valve, if it comes out of the oil filler cap you may have a bad head gasket.
