Testing coil cassette on 1.4mpi - correct method?

Just wondering if I've done this right?

I'm a novice who finally got fed up with paying mechanics to do nothing and decided to get haynes manual and have a go.

Have fixed a water leak and then moved onto engine management light on.

Got diagnostic cable and found code 304 (misfire on cylinder 4) on car computer.

Unplugged coil cassette, swapped and replaced spark plugs and still had misfire.

Unscrewed the coil cassette again and took out all the spark plugs and stuck them into the coil cassette.

Wrapped wire around all the spark plugs and earthed to the engine.

Started it up and, sure enough, no sparks on cylinder 4 plug.

Does this mean it definitely needs a replacement coil pack or could it be something else? I've assumed because the 1.4 coil cassette uses the 'wasted spark' method of firing two cylinders at once, it must be a coil problem as a different electrical problem would result in another cylinder misfiring too?

Is this the correct method to diagnose coil problem?