I'm not convinced you're using the right words here.. One might case harden to a RC40 (Rockwell Hardness) I'm not sure one can heat treat the material (4340) to a Rockwell hardness of RC40.
Heat treating can create a higher hardness the difference being that the core of the material is hardened as well. If the surface being re-worked is case-hardened by QPQ, nitriding, etc.. and needs a RC40 hardness then yes a re-case harden process would need be done.
Case-hardening or surface hardening is the process of hardening the surface of a metal object while allowing the metal deeper underneath to remain soft, thus forming a thin layer of harder metal (called the "case") at the surface.
So, the big question is how or what process was utilized the harden the material?