It can't have anything to do with an SD 4, as that's just a simple sound module without styles, an SD 2 with buttons. No, IMO it's clearly a successor of the SD 3.
Consider that Ketron had two bigger modules (or three, with the Audya) in parallel all the years: - a multimedia player named Midjay/Midjay plus -> succeeded by MidjPro - a style-focussed module named SD 3 -> now obviously succeeded by SD 40.