I've replaced odd sized threads by filling with silver solder and re-drilling and tapping with the required thread.
I also use thread lock for my brass props, available from garage or engineering suppliers or Halfords.

I have in the past threaded some brass bar and sweated this onto the prop shaft. Using a lathe you can then turn to the new die size and make a new thread. Make sure there is room for a locknut to fasten the prop.
If the shaft bearings are thick enough you could buy an M5 threaded shaft and make new bearings or ream out the existing bearings.
Dave

Solder m4 brass nuts to propeller

SHG used to do inserts years ago, not sure now. in engineering you can get insert threads a bit like a spring called, Heli-coils. But these are for replacing damaged threads and would be cheaper to buy new props.

You might need a two stage solution.

An adapter to go from M5 to M6 female:
http://www.wilesco-uk.co.uk/wilesco-Adapters.asp?part_id=101031

Another adapter to go from M6 male to M4 female:
https://www.thorlabs.de/thorproduct.cfm?partnumber=AE4M6M

Good luck.

Hi,
My immediate reaction is to get some M5 brass bolts and solder these into the threads. Then drill and tap these M4. You would need a lathe though.
Good luck
Edward