It is time to get a compression test. The engine needs to be warm. And, if you DIY, either the coils have to be disconnected or the plugs grounded - leaving the coils without the ability to discharge the electricity will fry them.

Because of the low mileage, one of the valves on #4 might not be sealing properly. The engine requires 2K rpm or more to allow the valves to rotate, ensuring an even seal with the valve seat. Driving the car up to 4K plus several times just might help.

You might want to consider a Seafoam treatment. The seafoam is sucked from the bottle - very SLOWLY - into the intake manifold via a vacuum line connected to the vacuum tree, until the engine starts to die. Let it sit for 10-15 minutes, then restart the engine and blow the loosened carbon out. It does smoke a bit :)
Do NOT allow the liquid to be sucked up rapidly, otherwise a cylinder can fill itself and liquids cannot be compressed - destroying the connecting rod and requiring a new engine.

Klaus
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The more I learn, the more I forget. So why learn?