Anything? That's pretty basic criteria there...Fusion, Malibu, Civic, Accord, Camry, Focus, Cruze, another Altima, Sonata, etc. Test drive everything and get the one you like best. 4 banger/4 doors/auto/$15K gives you a lot of options, so I would try them all if this is a car you want to live with for a while. Maybe even look at more interesting stuff like used Euro cars if you can deal with the potential quirks.
If I had $15K for a car and needed four doors, I'd probably end up with 10 1992 Explorers. Or maybe a early 2000s F-150 Supercrew and a few Explorers. Your options are wide open with such limited criteria at that price.