Couple of days ago, my car wouldn't start. Lights turn on, radio works, etc., but just a click when I turn the key.

Figured the battery was bad (it's about 4 years old) so we jumped it.
Took it to Autozone and bought a new one. Plopped the new one in and it started right up. All is good in the world.

But... I go to my car today. Same shit as before. Lights on, but just a click when I try to start it.

Is there another possible problem? It's hard to believe that I got a defective battery that goes bad just after one day.