I have googled this boat, and can see it is a fast RTR. The prop sticks out of the back on an outboard Z drive arrangement, like a Robbe or Graupner fitting, however it is likely not to be the 4mm that these items are.
Paul, as suggested by Malcolm, a few measurements need to be taken of the shaft size, and the method of prop attachment noted, ….screw thread or dog drive.
It may be the supplier of your boat has spares, or another supplier that supplies this craft can supply a prop. (did I use enough "supplies" then?)
Ashley