When they talk about air hardening of 4340, they may actually mean normalizing as opposed to a full austenitizing treatment and air cooling as we commonly think of it on air hardening steels. 4340 does give excellent and very consistent results when normalized. IIRC, the hardness will be around 39c, and intricate parts don't distort like they do when oil quenched. Lots of transducer bodies are made from 4340 and subsequently vacuum normalized prior to application of strain gages. Transducers have certain highly stressed areas of necessity, and the non-deforming characteristics of normalized 4340 makes it an ideal material for these parts. If the parts have internal cavities, they can be packed with steel wool prior to normalizing in order to give the parts more characteristics of a solid block as far as cooling is concerned. This will reduce any tendency to distort even further. We designed and built a 3 axis force measuring hub using 4340 in this manner. The transducer body started as an 800+ lb chunk of 13" round stock and ended up as a part one man could easily carry. Held our breath when it went to the heat treater, but it came back dead on the hardness numbers and with essentially no measurable distortion. Fun project