Thanks! The -4 functionality is important in this neck of the woods—roughly 5 months out of the year.
While minus 4 should suffice for most people, especially since the real problem would be less with a handheld flashlight that goes in and out of your pocket, than with a head lamp that would be fully exposed to low temperatures, possibly for a long time, as long as you stay with the aa/aaa format you can keep on hand a few Energizer Lithium batteries which handle minus 40F, they are expensive but have a 10 year shelf life.