"Folded in four" is ambiguous because I do not know what you are talking about. I can fold any napkin in four, so does that mean any napkin will be okay? "Four for four" is ambiguous because I do not know which is napkin and which is money. It would be clearer to say "Four napkins for four dollars" or "three dollars for five napkins". "Great in green" is correct only if there is a specific color called "Great in Green". Otherwise, if you are only looking for green napkins, you can say "I prefer green" or "green napkins would be great".