So I know these things are BEASTS for making fre tboards with the appropriate blade. The one in stewMAC is $80, so I said **** that and found it cheaper. It's a 4" blade and my Radial Arm saw is 10" or 8". Assuming the arbor is correct, can I use the smaller blade on the radial saw?

If so, that'd be sweet. I could sell everyone really cheap fret boards then!
I work with many different types of saws. Technically, you might be able to do it, but I wouldn't recommend it. Saws are built for specific types/sizes of blades, so better safe than sorry.