How would you write 4.315 in Spanish?

It depends on whether you are using common Spanish number punctuation or the English system.

Written in Spanish (in most countries), 4.315 would be "cuatro mil trecientos quince"; in English, it would be "four thousand three hundred fifteen."

Using English notation, 4.315 would be "four point three one five" or "four and three hundred fifteen thousandths" in English and "cuatro punto tres uno cinco" or "cuatro y trecientos quince milésimos" in Spanish.