I think 4U servers are quite rare these days, except for models designed to accommodate large numbers of either GPUs or 3.5" hard drives. Most 2-socket servers with up to a few dozen SSDs are 2U designs.
Agreed. My last company we used a 1RU with 2 blades (servers) in that space. Front to back cooling, quite loud. No need to worry about the noise in the DC. Power density in a rack vs computer density is all that matters.
