What could cause a 2001 Ford Focus DOHC with good spark plugs and wires to misfire in cylinder 4 when driving and cause stalling stumbling and P0304 CEL code?

Answer 1

Have you checked the injector? Try switching the plug and/or wire to another cylinder to see if the misfire travels with them.

That check engine means cylinder 4 misfire so I would still check the gap and make sure that the spark plug is good even though they are new another suggestion is the coil pack.

And a much less likely suggestion is that you lost compression but that's easy to test just take cylinder 4 spark plug out and put a compression gauge in and manually turn the motor over by the crankshaft.