These are usually coordinated modules with almost identical properties, often from one batch. With 4 modules, there is a lot of scatter and thus a lot of waste. But 4 modules make little sense in a dual-channel board (which is what almost all boards are today).

I don't know, then you just take 2 x 2 bars, then it's right again.

The price is apparently set by Corsair. At least the price difference in their shop is identical.