It's the one with the hose to the resevoir like I said. Some S4's have this on the radiator, some have it on the filler neck. It should be a 13psi/0.9bar cap, and unless you have a leak somewhere, that's how much pressure should be in the system when it's warm.

 

I have indeed replaced that part as well to no avail. I'm gonna take the car to get a pressure test done this weekend (having the car 100 miles away fawking sucks). I'll let you guys know about the results of that. I will probably have the shop that does that do a pressurized flush/refill if possible as well. Thanks for the input everyone!