I don't think it's 3 & 4. It's the injector closest to the left rear of the car (by the fuel pressure regulator), & the injector closest to the front right of the car (by the battery).
Fuel filter only has about 400 miles on it, I had replaced it a month ago trying to figure out why my injectors weren't firing at that point. I have no more grounds or contacts to check or clean.
I ordered another computer/wiring harness & will re-tackle this over the approaching weekend. Weeeee.