That 4th sensor can be reached from the top of the engine if you remove the coil pack first. If you put your hand down there (engine COLD, of course) you should be able to feel it. You might also be able to see the wiring harness it plugs into if you look back there, but you won't be able to see the O2 sensor from the top.
You'll need a 6" extension and a u-joint, as well as the O2 sensor socket.
Brian
