I haven't run the numbers, but I would imagine that 4 boxes with 256 GB of RAM are significantly cheaper than 1 box with 1TB of RAM, especially if the rest of the box isn't top-end equipment.
The density of the DIMMs is where the price difference jumps. Right now 32GB sticks push the price way up. Common current chassis have 24 slots letting you hit 384GB total with no big surprises on 16GB sticks.
Getting to 1TB would certainly require 32GB sticks and would be the point where multiple machines gets cheaper.
