A petrol will fire even with very low cylinder pressures. Check that each cylinder is sparking. A stout earth strap to the removed plug will forestall any overload on ignition coils.

I'd say it's a fuel injector too. Petrol fuel injectors are simple coils and can be checked easily be measuring their resistance. Disconnect the injector and use an ohmeter to measure the resistance at the two pins. I can't remember the exact figure you should be looking for but it is quite low (10 - 20 Ohms rings a bell). If one reads open circuit then it's faulty and needs to be replaced. A word of advice though. Replace the 'O' ring seals on all four injectors as it's quite likely at least one will split when you remove them. You don't want to be delayed putting it back together for an 'O' ring.