welcome to the site before anything what kinda of riding do you do?
a 4mil is a crank that goes 2mm higher and 2mm lower thus giving you more power and for a 421 kit you will need to get a 4mil crank, bigger carbs, a cool head, better pipes, reeds, pod filters, change gearing. lockup with cover, you might want to get it ported and also make it run on race fuel, ohh and also if you want just a little more power why not just port it and put a 4mil crank? you will save alot money by doing that.