very good information...
from what i have learned (Materials selection class) , when these parts are heat treated, the austenitic phase converts to a much harder brittle phase(martensite, and some others depending on quench rate and alloying elements in the metal) some of the austenite still remains. as the metal cooled, it reaches the martensite start temperature (TTT diagram) and then continues to cool until it reaches the martensite finish temp. sometimes this finish temp is much below room temperature and the result is some retained austenite phase in the metal. During this process, if the temperature is forced to decrease enough(cryo) the phase transformation is complete.
like already mentioned, there are also benefits from stress relaxation within and among grain boundaries as well as the formation of very fine carbides.
what is interesting to note is that this is 1 step in the heat treatment process, so sending parts to be "cryo'd" wont help unless they fully normalize the metal and start over with a new heat treatment cycle. without them knowing the EXACT composition and history of the parts you send them, they can't perform a proper heat treatment.
One advantage of 4340 alloy is that it has the ability to be through-hardened, meaning that thicker sections can be hardened than a regular high-carbon steel where only a thin surface layer can be quench hardened.