because there is no real new technology inside the SD4 it's just a SD3 upgraded, the extruder is the same(an horrible puzzle), the bed is also the same, same problems with the z axe(backlash), the SD3 is at a lower price and with the difference you can buy some real upgrades for the printer(The E3D head for exemple). Take a closer look a the two printers and you will see that there are not so much evolution, expect the filament support and the box that have been changed all the rest is quite the same.