2003,How do I tell what size lug nuts I need for my rims?

So I was looking for rims for my car and found ones that I liked. My rims have a 4 bolt pattern and these other rims have an 8 bolt pattern. I was wondering what the lug nut size for my rims are? The person I might by the rims from said that the lug nut size are either 4×100 or 8×100. I don't know what that means so that's why I'm asking.

My car has 143675 miles.
My car has an automatic transmission.