MK3.9 contains everything from the MK4 except for the X/Y/Z motors ( Nextruder motor is included ). You will reuse the frame, PSU and motors from your MK3S+. And if you want, you can additionally buy the new precise MK4 motors (eliminate Vertical Fine Artifacts, 0.9° per step) and complete the upgrade to MK4 later. This product is supplied as a kit - you must assemble it yourself!