The inserts of the spiral coupling are not threaded.
They are secured with grub screws.
That's why I milled depressions in the shaft to prevent any slippage.
See my 2nd pic below with the shaft and coupling on the mini milling machine.
Use a flat ended mill bit to prevent it skidding off the shaft - like spiral drills would😠
I also fitted a collet in the same way, visible to the right of the pic.
I then fitted spacer washers between the collet and the end of the shaft tube.
Prevents the shaft and prop exiting stage backwards if the coupling should come loose.😭
Which mine never has 🤞🤞
3.94mm I would say is well within the tolerance of a 4mm drawn rod, especially those produced for the model market, and not for a NASA rocket 😉
I assume that your prop shaft is only threaded at one end.
That's where you screw the prop on. DON'T forget a lock nut and a thrust washer👍
The plain end goes into the coupling. If you can't mill a depression into the shaft for the grub screw just file a flat. Secure the grub screw with Loctite or similar.
Hope this helps.
Any doubts just YELL.
Cheers, Doug 😎
PS Just added two pics re milling the prop shaft👍