#4 Fuel Injector Not Firing-Tried Many Things, Can't Figure It Out-
 

I have a 90 240sx with the KA swap. She ran great for 6 months after swap and randomly about 2 weeks ago it started missing. I diagnosed the prob to the #4 cyl. I checked the compression, spark, and they're both good. I thought it might be the fuel injector, so i bought a new one and still it changed nothing. I'm guessing it is in the wiring??? The ecu is throwing code 34, knock sensor. Anybody have this kind of problem or have any ideas on where to start looking for the answer???
Thanks