I would consider using 12 batteries total. I am running 12 * 1.2 = 14.4 volts. That's a bit much for a 12volt bulb. But they are rated for thousands of hours at 12 volts, so even if mine last half that I am still happy. There only $5-6.
Plus when pulling 50 watts through AA batteries there should be some voltage drop. I have not measured though.
Could you use 4 holders in a 4 D?