4K is "liquid helium" temperature, as used in medical imaging. It is *relatively* cheap. Sub-1K is achieved by an out-of-equilibrium mixture of Helium-4 and Hel

4K is "liquid helium" temperature, as used in medical imaging. It is *relatively* cheap. Sub-1K is achieved by an out-of-equilibrium mixture of Helium-4 and Helium-3 (a much rarer, very expensive isotope). This is why sub-1K is much more expensive than 4K cryostats, for a given cooling power.