4140 steel is a chrome-molly steel, with .4% of carbon. That .4% carbon content will allow the steel to harden somewhat. Not much, but somewhat. The Chrome content combined with the mild carbon content will allow this alloy to harden more, and more deeply than, say, 1040 plain carbon steel.
I recently bought some raw cast parts from Jerry DeVaudril that were made of 4140 steel. Jerry referred me to Bob Roller for heat treating advice.
MR. Roller hardens frizzens cast of 4140 steel by quenching in 5w-30 oil, but does not temper the face of the frizzens.
Roller locks are well known for their quality, so he must know what he is doing.
8620, on the other hand, is a great steel for case hardening, but has a reputation for warpage. Either steel could be used for lockplates, with the 4140 being more durable, but 8620 being easier to work with when it comes to machining and color case hardening.
Just kinda thinkn'....typin' out loud, so to speak...type.
J.D.