Connect this to that with a set-screw coupler. These couplers are made of machined aluminum and have a spiral cut that makes them slightly flexible so they can be fit to two shafts even if they are not perfectly co-linear and will help reduce binding effects. The way they’re cut, they’re a little springy in the X Y & Z linear axis but not in the rotational axis. That means that you won’t get increased backlash – so they’re suitable for precision CNC work. This coupler will connect a 4mm diameter shaft to a 6mm diameter shaft.