Just wanted to chime in to say that my SP4 pen mysteriously stopped working while taking some notes during a meeting on Monday. I went without my pen for 2 more days while waiting for some AAAA batteries to arrive from Amazon (got 8 for $5.40 CAD) and just changed the battery now. Pen is working again. I got my SP4 at launch on October 24, 2015, which means that my pen battery lasted 1.5 years. Anyone else had to change their pen battery yet?