The only difference is that there are four EGR(exhaust gas recirculation) ports in the head above the exhaust ports. In order to make an '03 header work on an '04 you have to take a 1/8'' pipe plug Tap and tap the small ports for the EGR. Once tapped you need four stainless steel pipe plugs, srew them into the head; if they don't start by hand screw the tap back in and make sure the threads on the tap are clean because the aluminum will clogg the threads easily. Screw the plugs in by hand and use a small box end wrench to tighten, this will help prevent overtightening because the aluminum is very soft metal. They don't have to be extremely tight but snug then install the new header for an '03 you can re-use your gasket and your done. Also some people try to move the engine to get the old header out but it slips through almost without a catch if you remove the passenger side axle just make sure you drain the transmission first; theres a 17/19mm plug on the bottom facing the front of the car. Its a little more expensive because you have to buy new trans lube but a lot easier in the long run! I used Lucas 75w90 synthetic it was like $36 but its good to change it anyway! Now you know my secret to making this part work on my car!! You also have to remove all the EGR crap, its the turbo looking thing and there are a few wires to diconnect; This Will Set a Code!