400 lumens is a lot to expect from AAA batteries, that's a 5+ watt led unless it's cutting edge, and even still 3 watts.
Honorable flashlight should have a SEPIC / boost converter to operate more independently from the cell voltage.
I built a ~600 lumen lamp for my mountain bike, operates off 4 AA nimh, ~4-5 volts in, ~9 volts at 1 amp out... get an hour or so at full power, but usually run it at half power for longer rides, using 3000 mah digital camera batteries (capacity is pretty close confirmed by Wizard One charger)
that reminds me, need to find that lamp so it can get packed for my upcoming trip.
