your post's a little confusing cos #4 is nearest the bulkhead!

have you replaced the plugs?

if you're sure they are ok and the problem is with injectors pull the plug leads and crank it over. does it sound like there's compression on all cylinders whilst cranking?

then pull the plugs and see if there's unburned fuel on all cylinders since if any injectors are not opening there will be none in that cylinder.

then replace plugs, fire it up on the 3 working cylinders again to burn off the residual fuel from the previous test. then swap the injector connectors between an adjacent 'good' cylinder and the 'bad' cylinder -this will help to determine if the problem is with the injector or the electrical connection to the injector (or the ecu)