I agree. 1 box for evey 4 hens.
As far as how high, people do a lot of different things. Some have them on the ground, some quite high. Usually, the chickens don't care that much. Some do but usually not. So make it convenient for you.
Some of the things to think about. Chickens are ground dwelling birds. If your coop space is tight, putting the nesting boxes high enough for the chickens to get under increases your coop space.
How much do you want to bend to get the eggs?
The usual recommendation is to raise them about 18" to 24" so they stay cleaner. Nest boxes on the ground are more likely to get dirty.
Your roosts need to be about 12" higher than your nesting boxes so they are less likely to roost in the nesting boxes. Do you have room for roosts that high.
I think those are the major considerations. Maybe someone can add some more things to consider.