Deep (50.8mm) threading M36-4. What's your first choice to do the job.
Can't be turned. Can be milled.

I have some tiny little micro boring bars I use for deep thread milling
small threads, but a 4mm pitch needs a huge tooth... and of course the
reach to do the job.

Not my project. Operator would prefer some form of thread milling. The
work piece was not described.