Is 420 stainless steel stronger then 440 stainless steel on knife?

420 and 420J are low quality steels that are usually considered incapable of being used to make a quality blade. 420 is soft, tough, doesn't hold an edge very well, but used in some diving knives because of its stain resistance. 420HC is roughly equivalent to 440A and is also considered an inferior steel. It is possible to heat treat 440A to produce a good knife. SOG has used 440A in the past and may still be using it (I don't keep up with the steels various manufacturers use). 440B is also capable of producing a superior blade with proper heat treating. Randall reportedly uses 440B for their stainless steel knives. 440C was the top knife steel during through the 1980s and continues to be used to produce superior blades

420 Steel

Generally no. But when it comes to 440 there are several grades of it. Most of what you see in the knife market if it's cheaper is 440A which is basically crap that cant hold an edge. Your going to want 440C or higher. Those knives you paying $60 starting.

for stainless the lower the number the "harder" the steel will be (harder not stronger) the harder the steel is the longer it will hold and edge but it will also be more difficult to sharpen.