"superposition of stresses" is an engineering principle that we use for lots of things. That in conjunction with prestress in the spokes when the wheel is built is the key to the wheel functioning. It can be a hard principle to grasp- whether you think of the spokes hanging from the top or carrying compression from the bottom or whatever, it all gets back to this same principle. The completed wheel is complex and applying load at one point on the rim can affect the stresses somewhat in all of the spokes.

As for the "4 spokes at the bottom", this also depends on the particular rim section you are using. I.e. some deep rims are stiffer than some of the older thinner rims, and will distribute that load at the bottom to more of the spokes near the point of contact. It was mentioned above that the rim will try to oval-shape when it touches the ground, it will do this a little less if you have a deep v section rim.

I'm not up on the latest wheel building technology. Now that we have spoke tension gauges, what tension range is typically used now? I would guess that a single spoke could be pretensioned to 150# or more, so "the 4 spokes at the bottom" would amount to a lot. As a structural engineer I would like to read Mr. Brandt's book, but it seems a bit expensive to me for a casual read. (Maybe I shouldn't be so cheap.)