They may cost the same, but there is a LOT more you are missing on the 4mil. You can't just buy the crank and throw it in there. You will need spacer plate or stroker domes, if you don't have a cool head, you will need your stock head machined for the stroke. If its a longrod crank you will need new pistons. You should get porting to take advantage of the added stroke. Longer swingarm, new chain / sprockets... list goes on.
There's probably some other mods out there that would wake it up more than bigger carbs or a stroker crank. Check out some porting first. I would def. go porting before I did a stroker crank. Its cheaper in the long run and still give you good gains.