16v running on 3 cylinders

[derrickmd18@juno.com (der flo)]

I have a 16v with a running problem.  when I start it its idles rough. 
when I adjust the idle screw it does nothing.  I can adjust he screw all
the way out and in and it idles the same way.  I when it warns up it runs
on 3 cylinders.  #4 cylinder won't fire.  I changed the spark plugs and
wires and still does the same thing.  does anyone know what the problem
maybe.  should I change the coil next?