You can go a bit lower than the temperature of liquid helium just as you can cool something to lower than the temperature of liquid water just by using evaporating water. However, 4 Kelvin is probably correct. The image they show appears to be of a cryostat that would almost certainly use helium as a cryogen. Such cryostats are realistically limited to around 2-3 Kelvin. They can go a bit lower when empty, but generally not if you stick a complex experiment with a lot of heat-conducting fibers and wires into them.

4K is probably warmer than they'd like for a quantum circuit, but getting their apparatus colder would require a much beefier cryostat or more exotic technologies like magnetic cooling built inside a helium cryostat. Such setups are capable of getting complex setups down to the hundreds of millikelvin, which is much nicer for this sort of work!

As for writing a scientific article using Fahrenheit... Please don't do that. Just don't.