You are running a lot more camera than I have ever used so this may not be of much use. But, for what it's worth:
I think you can use an f4, but I probably wouldn't if I had other choices.
I love shooting the MW and use a D750 with a Rokinon 20mm 1.8 (@1.8) or a Rokinon 14mm 2.8. You have to manually focus these things anyway and on my body the Rokinon seems to do a good job. At 20mm I think I can get away with close to 20 seconds exposre, depending on where in the sky I'm shooting. I say "close" because I use the "black hat" method of shutter control and a 1 mississippi, 2 ... timer. In the image below, you begin to see some star trailing at about a 50% crop. That may be real trailing, or my focusing may have been a little off.
View attachment 58717
I used the in camera long exposure noise reduction and it seems to work well on my D750. If all you have is an f4, go for it, but bump the ISO a couple of stops rather than extending the exposure.
Also, in post I've found that the brightest areas in the images (just below center in the example) can try to blow out the dust lanes and other parts of the image. This is pretty easily tamed in post.
Good hunting. I'm envious.