It's just one cylinder not working so it kind of narrows things down a bit..
1) Work out which cylinder is causing the problem.
2) Take the coil pack off that cylinder and peep down the plug wells.
2a) If there is any oil in the plug wells (#1 and #4 seem to be the worst culprits) dry it up with a cloth or tissue.
2b) Take the plugs out, make sure they're clean and in good condition and that the HT cap and lead all connect tightly.
If you fixed anything - start the car and see if it runs better.
3) If no joy, take the coil off and swap it with another. Check if the problem moves to the new cylinder (that's your coil is shagged) or stays on the original cylinder (it's something else).
4) Check the voltage to the coil on that cylinder when the car is running. Is it the same as the others, or is it zero?
5) Check the injector on that cylinder is getting a "Pulse" when the car is running.
4a & 5a) If the coil and/or injector are not getting any juice, you need to investigate further where the "break" in that particular circuit is.
6) If necessary (desperate) swap the injector between the duff cylinder and one of the others. Does that move the problem (duff injector) or does it stay with the original cylinder? Don't test the injector by running the car with it out of the cylinder.. (unless you like fire).
7) If everything checks out, do a compression test on the cylinders. Is th duff cylinder the same as the others or way lower? Could be a compression loss (rings or piston). Did you drive through a puddle, or wring the beast's neck recently?
My guess is you have oil in the plug wells (clean all of them) or worn plugs (since they're expensive long life ones that don't last quite as long as Alfa says and nobody replaces them on time anyway, or fits cheap non-long-life ones) or one of the rubber caps or HT lead have a corroded inner connector so the coils are not feeding spark to the plugs).
None of these are toooo expensive so don't panic just yet..