Have a look if the inlet manifold is cracked , if its running on 3 cylinders and the manifold is leaking it sounds like there will be a crack that is loosing all the air to that cylinder so that cylinder wont fire, i had a car the other day at work running on 3 and took the coil pack off and there was air coming from the spark plug, so removed the spark plug and the spark plug inner had blew out so air was just coming up threw the middle of the spark plug, check them aswell but more seems to be due to a crack.
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