From what i hear, they did a change with the cats, supposedly they are a little slower than what a 03 would be.............heard that on um, hell i forget. On another site, dunno though.
I was looking at some 2004 Mustang GTs on a dealer's lot, and they all had a different badge on the side. It was an anniversary badge, and looked just like the V6 anniversary badge, so I don't like that about it.
Other than that, it looks the same on the outside, as far as I could tell. Same fake hood scoop, same fake side scoops, perhaps the rear badge still said "GT" (I don't remember for sure).
Are some differences with the cats adn O2 locations. Had to modify my sensor wires to fit with the SLP x-pipe for an 03. The 40th badge is only on a run of about 5000 cars... mine has it. The 40th also has the spoiler delete feature.
I could've sworn the trunk deck was a little shorter on the '04, it may have been my imagination though. Either way I finally got a job that can pay for it so I'm buying an '04 GT this summer