8 bit is not sufficient for BW, I have seen that over and over. If you need to increase contrast much, and you WILL with BW negs if you don't want the typically flat, lifeless image that people post here from their scanners. Use 8 bit, and give the pic normal contrast, you'll see posterization. I promise.
4000 DPI is overkill and a waste of time until the day that you take a lower res file, spend a ton of time post processing, retouching, doging and burning and making it perfect....only to decide you want to make a large print. Oh hell....need to rescan and do ALL that work again. I rarely print my files the full size the 4000 DPI allows, especially from my medium format scans...but the times people have contacted me to buy a LARGE print, I was happy I had done the smart thing and scanned everything at max resolution.