Stainless steel has at least 10.5% Chromium, and the maximum carbon content is not more than 1.2%.The main characteristics for these kind materials are good rust resistance and corrosion resistance.
It is clear that AISI 4340 is not stainless steel.
AISI 4340 is defined as a heat-treatable low-alloy steel containing 0.8% Cr, 0.2% Mo and 1.8% Ni as strengthening alloying elements. Compared with Stainless steel,it has higher strength and toughness, but its atmospheric corrosion resistance are not as good as stainless steel.