My first question should have been whether or not it has a lope. You said the idle is low, but is it smooth, or rough? If it's low, but still smooth, it's NOT a spark plug (unless all four happened to degrade magically at once). If it's smooth, it's something that affects all four cylinders. Usually fuel delivery system is the culprit. Could be a fuel pump, but a clogged filter is about 10x more likely. It's not the alternator .. alternator won't do that. But if it makes you feel better, have it checked anyway.

All I'm saying is that smart money says check your fuel system. And you don't HAVE to go to a dealer, by all means, replace the filter yourself if you know how. Throw in a bit of injector cleaner while you're at it. What have you got to lose anyway?