Ok it's not an impreza but my clio lol.

Sounds like it's on three cylinders. Engine note doesn't change if I take the lead off the coil pack from cylinder 4, but it does change if I disconnect the injector for cylinder 4. I just changed the injector for 4 'cos I assumed it was this based on tests yesterday, but no good. I have had a look at the plugs and they looked ok, but I suppose I should think about leads and maybe coil pack now, but I'm still confused about why the note doesn't change if I take the lead off the coil pack? It 'seems' there is combustion with fuel and no spark? HOW!?

Edit - Fixed.

 

HOW!? It's called detonation and it's how diesels work. Not so good on a petrol engine though. That said if it were you should hear it pinking.

If disconnecting the coil pack on cylinder 4 makes no change my money would be on that but I'd get someone to check the electrical pulse going into the coilpack and assuming there is one then whether or not it is causing a spark. You might be able to check it with a multimeter on tick over.

 

coil pack i would put my money on

 

It was the injector all along, I just got my cylinder numbers wrong lol. As soon as I typed this post the penny dropped.

 

i was gonna say injector. its common fault along with coilpacks on these cars. i had number 4 injector go on my 172. you should be able to pick up a faull rail with injectors second hand pretty cheap