I designed this tool to remove the 4 mm nuts on my WLToys K989 RC touring car.
In PLA this small tool does the job very well.
Print four, one for each wheel. You can keep the nut in the tool (which is easier to find when it is time to fix the nut in place again).
Hex socket is designed to 4.1 mm.
A fine resolution is required as the nut itself is quite small.
On my Ender I had to scale up very slightly (2.6%) for a perfect fit.
Let me know how yours works out in the comments. If you are unable to scale for a size you need let me know what you want and I'll add it to my CAD list.