Autogeek carries a 5" BP and a 4" (3.75") BP for rotaries, if you get both, you can use all of your pads with either machine. I would also get a larger BP (5.75") to use with 6", 6.5", 7.5" pads, including the LC Purple Foamed Wool pads. That is the setup that I have, a 4", 5", and 5.75" BP for both machines. You never know when you have to use a smaller pad on a tight area or something. Sure it'll cost a little more, but if you already have the pads, you might as well pay for the flexibility.