I need to reduce the speed of my 1/2 horse electric motor, that is turning at 3470 RPM.
Using a belt and pulleys, I need a maxium speed of 850 RPM. That is the speed of the band saw that I will be running. Am I correct to think that this roughly a 4 too 1 ratio. And I can simply use a 2in pulley and an 8in pulley? Thank you very much.

Yep, but your also going to have to pick a belt.

Pulley And Belt Information Calculator (http://www.csgnetwork.com/pulleybeltcalc.html)