I think your maths is borked.
4x1.5= 6v (in a series)
I used to use 8AA hooked up in a series (4x4) to power my 12v fans, but I found that the increase in weight was just ridiculous. I have since switched to 2x9v batteries which still gives me over 12v, but reduces the amount of weight. AFAIK, undervolting a fan will cause the battery to drain faster.