This has been a very interesting discussion. I know bolts are quite weak in shear, but I figured 6 of them on a 4" wheel would be plenty strong. Considering the holes in the wheel and sprocket are about 0.206", my suggestion would be to replace three of the #10 bolts with 5mm roll pins. Something like this:
http://www.mcmaster.com/#roll-pins/=eqsixn
Roll pins are inherently oversized, so the actual diameter of these would be .212" - .220". They should fit nicely in the wheel and sprocket, and take all of the shear loads, allowing the remaining three bolts to simply hold everything together.