Another test you can do is to apply direct voltage to the injector. Grab a voltage probe and connect the leads to the battery, then remove the injector clip. Touch the positive to the pos. terminal on the injector, and the neg. to ground, and you should hear the injector fire off (clicking noise). If it doesn't fire, then the injector is bad. If it does fire, then you have either a harness, ignition, or injector driver (ECU) issue.
I'm not sure of the amount of injector drivers on the ECU, though. I don't know if one controls all four, or if they're individually controlled.