LP-E4 Availability

I own a 1D Mark III and 1D Mark IV which I plan to keep for as long as they continue working.  I have the two original batteries and one spare that I bought when I got the Mark III, so one battery is about a year old and two are three years old.  All batteries show Maximum Recharge Performance. Early next year   I am going to Hokkaido and will be photographing wildlife in sub-zero temperatures. Should I get a fourth battery to ensure I will have enough power under cold conditions?  Or get it to make sure I have enough batteries in case Canon discontinues the battery?  (I've noticed that it is becoming more difficult to find Canon LP-E4 batteries.)