It means the #4 cylinder is misfiring. Can be bad plug, plug wire, burnt valve or blown head gasket. Replace all the plugs and wires and see if the misfire goes away. If not, run a compression test on all cylinders.
Cylinder #4 is misfiring in an engine with 4 or more cylinders. Engines have a firing order that is used to number the cylinders and it depends upon the engine which one is #4.
Can be a bad plug or plug wire. Can also be a burnt valve on that cylinder, or a blown head gasket. Run a compression test on all cylinders. If cylinder #4 is way below the rest, suspect an internal...