Four indeed.
In the late '90s and early 2000s the Cateye Micro was the most popular bike light on long-distance rides. It took four AA batteries yet it was still really compact. It had a tiny 2.4 watt halogen bulb and produced a small white blob of light. Many people (me included) mounted two on their handlebars. Twice the light - luxury!
However, on alkaline batteries you'd only get, at most, a couple of hours of usable light from them. But Cateye had a solution for that - an
external power pack that took
four D cells!!! It was literally the size and shape of a drinks bottle and was designed to be carried in a bottle cage.
I was the only person I ever saw with one.
They weighed a (metaphorical) ton but would indeed give an entire through-the-night worth of light. Even so, it was still just a small blob of white light.
Kids today, they don't know they're born...