Bob the birdhunter is correct that 4140 alloy steel is not to be used for normal high temperature quenching color case hardening(CCH), as it is a through hardening steel. The 8620 alloy steel that you mentioned can be CCH, I believe.

However, that being said 4140 alloy steel produces beautiful coloring when Turnbull uses their modified approach to CCH for it specifically, and there appears to be scant surface hardening, or at least that is my observation on the work that Turnbull did on the 20 bore DR they CCH for me. See this link to the "Paradox" forum: http://forums.nitroexpress.com/showflat.php?Cat=0&Number=225579&an=0&page=0#Post225579

Transvaal, AKA Steve