A lot of people have been suggesting using the smaller 4" pads for some ares they aren't haveing too much luck with the larger ones with, like some sirlies, scratches, etc. The theory behind it makes sense, the smaller diameter focuses in one spot more, or something like that. But I would certainlys tart with larger pads first, take slong enough the way it is, couldn't imagine doing a whole car with 4"!