In the last 3 months my 2004 wrx has been suffering an intermittent missfire sometimes on cylinder 4 sometimes on cylinder 1 I’ve swapped coils put in new plugs 

it’s currently got ngk iridium plugs and surefire coil packs all were new 6 months ago all working fine then this happened.

im at my wits end with it now 

I reaching out as I’m very close to just scraping the car as I and a mechanic can’t find the fault 

kind regards 

Ben 

Try swapping the two misfiring cylinders coilpacks over to the two cylinders that don't misfire .

If the misfire doesn't change cylinders when you swap the coilpacks to different cylinders and spark plugs are the right heat rating/ gapped right . Here's a few things I'd probably try ...

Check and clean the engine loom earths and the multi plugs at each end of the loom. Also main engine earth on starter motor bolt .

Check securing tabs on coilpack plugs are working and they're a nice snug fit (plugs dont wobble on the packs ) .

Check pins on coilpack plugs are clean and free from corrosion .

Check the Tgv's are operating properly. 

Check ECU grounds are making good contact. 

Maybe a smoke test to rule out a intake manifold or inlet track air leak .

Hopefully its something simple and best of luck with it,  fella