Just received my 2 new backup chains from Amazon. They are actually Poulan factory and not generics. I thought they were a pretty good deal at only $15.07 each and of course Prime free shipping. You can spend almost that much getting a chain sharpened!
Welcome aboard JJP in Ohio I run a 4218 myself. Little long in the tooth now that it's around 18-20 yrs old, but still cutting. Edit.. I usually run these, as they're local and it's near my work. Granted, those won't work on the 20" bar.