That would be the case if the CPU had four memory channels, but consumer platforms only have two. When you install four modules the bandwidth is shared between

That would be the case if the CPU had four memory channels, but consumer platforms only have two. When you install four modules the bandwidth is shared between each pair, so on paper you end up with the same bandwidth as if you used two modules at the same clock, but running four modules puts additional strain on the CPUs IMC and demands better motherboard signal integrity so the clock speed usually has to be reduced to get it stable.