This is the Great Planes ElectriFly 4mm to 1/4-28 prop adapter and propeller nut.
Features: Large shaft size for Brushless or Brushed motors with 4mm shafts
Requires: Tightening the prop nut to compress the prop adapter onto the motor shaft
Total length: 1.3 (33mm)
Diameter of prop washer: 1.1 (28mm)