Breaker #4 in the picture is wired to my kitchen lights and living room outlets. It keeps tripping every few days. I thought maybe we overloaded our living room at first, but it tripped this morning with only lights and TV on.
Now I'm thinking it might be a bad breaker, but when I went to Home Depot to buy another one, there were no Square D breakers with that little white button on top.
Best Answer
That is a Square D QO series breaker. The one in question is an AFCI, or arc-fault breaker.
The early incarnations of QO AFCIs had some issues. There were blue, green, purple and I believe another color "test" buttons. How old is this breaker? An AFCI can trip due to electrical "noise" on the line, motor brushes like in vacuums, etc, any kind of arcing or sparking in the circuit, frayed cords, and more.
Here is a good troubleshooting guide for AFCI breakers right from Square D: http://static.schneider-electric.us/docs/Circuit%20Protection/Miniature%20Circuit%20Breakers/Arc%20Fault%20Circuit%20Interrupters-AFCI/0760DB0204.pdf
Also, more from Square D: http://www.schneider-electric.us/en/faqs/FA125415/
Here is another very good answer to the same question: https://diy.stackexchange.com/a/74242/19875