>The modifiers conferred by moves operate on a sliding scale of stages. When a given stat is raised or lowered, its current stage is increased or decreased by the amount dictated by the move, up to a maximum of +6 or a minimum of -6.
Thus, regardless of Weakness Policy or not, it will gain +6 (x4) Attack.
Tested it on Showdown as well. Weakness Policy + Belly Drum won't work.