Whilst it is very unusual for Ketron to remove a product completely, the SD4 is no more sadly. I believe remanufacturing would have been too expensive. Its a pity as it was an amazing little module that was not appreciated enough.
Used Solton and Ketron instruments are hard to find, most musicians keep them as back ups, or just because they have invested so much time in customising the way it operates they just dont like getting rid of them. Most Ketron owners I know still have all their previous models too!