A shed is a thought, but it would have to be at least a couple of feet taller than the tank in order to have access to the manhole on the top.

I would be doing rainwater collection off a metal roof. Filtering the runoff would keep most debris out of it. Without sunlight, there shouldn't be any algae growing in the water, and the kind that grows on the side/bottom of the tank is not a worry to me.

This would mostly be for watering the garden, and for drinking water if backup should be needed.

But 4" of concrete isn't really needed unless you're working with that watery slop that the trucks deliver. The more water added to concrete, the weaker it is.

I've been working with concrete for about six years now, and I know that when the concrete is made with minimal water, it's very strong. Most of my pieces are only 1/2-5/8" thick, and hollow. However, I am not sure about the expansion/contraction issues with the tank vs. the concrete shell.

There has been quite a lot of work done in Africa and the Middle East using Watt's ferrocement methods, and quite a bit of construction with thinwall ferrocement for roofs in temperate climates (including Alaska). The thinwall tanks do stand up to the weight of the water, but freezing temps are not an issue there, which they are here. I am suspecting that the bulk of the water (another reason for such a large tank) might offset any freezing issues for the temps we get here, but I don't really know.

Sue