The largest of the bichirs is Polypterus endlicheri congicus...which, I suppose, could be called a 'banded bichir'. Bichirs should have a varied diet. I feed mine...frozen silversides, frozen 'krill', frozen beef heart (occasionally), live earthworms, fresh seafood from the supermarket like shrimp, clams, etc., and pellets (I personally use the Hikari brand of sinking carnivore pellet but others will work). Their main food is the pellets but they get frozen or fresh food at least a couple of times a week.
Your 6 inch ghost knife (what kind of 'ghost' knife is it?) will be OK with a 4 inch bichir but if you get P. endlicheri congicus then the bichir will eventually grow to a large enough size that your knife fish might become a menu item.
-Joe