While the engine is running try disconnecting one injector plug at a time. If the engine tone/rpm changes then that Cyl is functioning, if the tone/rpm don’t change then that Cyl isn’t functioning properly. At least by doing this test you can possibly narrow it down to a particular Cyl.

If you do find out that it is a single Cyl, then it could be a number of things, faulty plug lead, faulty injector etc etc.

If there is no difference in the engine tone/rpm by doing this test, then it is likely what cossie604 suggested.