We feel that in order to have a visible difference when up sizing a breast implant we need about 150cc to make the effort "worth it". Certainly the change from 300 to 400 will not be massive as you will have to set your sights higher.
Since you are only increasing in 100 cc's, this is not a massive difference and may not increase a full cup size, depending upon your original size and frame. However, the pain would probably be much less since the tissues, especially the muscle, would not have to be stretched as much as in the initial surgery. Your surgeon can best guide you to what size you may achieve.