Seems to be a bonus, applied randomly. I've never seen a +4 to Yield though (which would be 5 items per gather), only +3 (4 items per swing).

The bonus can apply right up to ShB nodes but will only apply to items already at a +2. I opened a ShB node once where the lv71 item and the Crystal gained a +3 but the other items which are usually +1 stayed at +1.