Where did you get the notion that you should get 2 to 4 days on a charge? Most of today's smartphones will give you 12 hours or less of power, but of course, it also depends on how much you use it.
Also, you should not allow a lithium battery to get below 20% on any sort of regular basis. That will damage the battery, shorten its life and also could require more frequent charges.
My advice is to keep the battery topped up when you are near a charger and read this from our FAQ: Maximum Battery Life - BlackBerryFAQ