On-topic: Where the SB(2) has deck change buttons, the RB does not. This is not to say you can maybe do something in software, but that would require touchscreen or mouse action every time you want to change decks, so effectively not usable imho.
Near-topic: Even if you have the SB2 and you CAN switch decks, it means that every (mixer/deck) control you move, you will have to manually move back to where it was when switching back to the previous deck. Especially for the mixer part that is a pain I think. So I would never advise (regularly) using 4-decks on a 2-channel controller.
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