Originally Posted by Adam1222

Usually, when referring to 4 legacy carriers, the reference is to US Airways, American, Delta, and United. (Some people don't consider Hawaiian and Alaskan legacy carriers due to their regional nature.) The wikipedia makes clear that 4 is referring to US Airways: "There are currently four US-based legacy airlines left that operate transcontinental and overseas route networks. That number will shrink to three once American Airlines and US Airways complete their merger, most likely in 2015."
US Airways and American are one company now, but have separate operating certificates.