In this series of steel the second 2 numbers are the carbon content, so 4140 has 40% carbon content,about 10% more carbon than 4130 and 20% more than 8620. Carbon is what enables the steel to harden. If you do a good job hardening and tempering then 4140 should be just as good as 4130. 8620 is almost a mild steel as it is a low (relatively) carbon grade suited to case hardening. I looked it up yesterday and this is what I found: Principal Design Features - AISI 8620 is a hardenable chromium, molybdenum, nickel low alloy steel often used for carburizing to develop a case-hardened part. This case-hardening will result in good wear characteristics.
From the Fulton Armory website I learned that some M1 Garand receivers and bolts were made with 8620.
IMHO I think as long as you use Kasenit to harden fire control pin holes it should be fine for AK receivers.