A few terms that need to be understood. In heat treating parts which are to be hardened, are hardened to their maximum attainable hardness by being heated above their critical temp & quenching to cool them suddenly, the actual quenching medium depending upon the actual alloy. If this max hardness is not what is desired (Usually isn't) then they are tempered to bring them down to the desired hardness range. The higher the tempering temperature the softer the result. Charcoal & Cyanide are essentially two carbon bearing substances which when the part is heated above that critical temp the carbon is absorbed into its surface. The longer it Soaks at that temp in the presence of the carbon the deeper it penetrates. However the deeper it goes the longer it takes to go further so there is a more or less practical limit to the depths achievable. An alloy steel such as 4140 is seldom case hardened or colored. These steels are most often simply heat treated. They are quite often used for parts requiring high strength & the temps for coloring cannot be higher than the tempering temperature to which they were heat treated. I worked a few tons of 4140, 4340 etc in my career as a machinist & seldom died we temper it higher than around 900°F. Coloring it by cyaniding would be a specialized process not used too often in my opinion. Thos last two digits in 4140 denote approx 40 points carbon, case hardening is normally done on steel with a max of about 20 points carbon.
