4 AA or bigger cells in series should be OK but marginally so as the regulator wants to see about a volt more in than it's output voltage for proper operation, there is a diode in series with the DCin jack + connection and that leaves 6V4 - .7 (4 X 1V6 = 6V4) or 5V7 to the regulator which should work ok for all but the highest current loads > 300 400 mA. The Problem is that the batteries output voltage will fall during discharge. A new unused AA Alkaline cell will measure about 1V65 - 1V7 and is considered dead at .9 - 1V1. 4 X 1V1 = 4V4 and the minimum required voltage is 5V7-8. 3AA batteries connected to the 5V source on the Arduino or 5 AA batteries connected to DCin should work fine. If the AA batteries aren't big enough (enough current/time or AH capacity) you can always go to a "C" or "D" sized cell, the voltage will remain the same but the larger batteries have higher AH capacity and thus are better candidates for powering your project.
Bob