I mean, 2-layer could also work.
Yes but 4-layer is MUCH easier to design and the end result is better because of the extra ground and power planes. It does cost more though.
For arduino, that is a good way to protect themselves.
I would understand as well if that's the case, in fact I floated the same idea some time ago. I don't think anyone here would blame them for trying to have an edge, after all they did all the work. However it's not open source if that's the case.
Rob