PM motor why divide by 4?

I need a 48v generator for a win-gen.

I found a 4z266e Dayton PM motor. Old stock, but new in the box.
Rated 2500rpm, 180v, 11.5 amp, TEFC, Reversible
I guess the weight to be about 60 pounds.

I've read that dividing by 4 would give you the rpm and voltage it would begin to generate. Why 4?

How would I calcuate the size of rotor to be used?

I live on the border of a class 1 and class 2 wind zone.

Thanks for your ideas and wisdom,
Bill