Were there ever any 4 bit OSes? I can't imagine that there were. All the 4 bit powered devices that I know of didn't really have an OS, they just ran one application. That would be things like microwave ovens, TV remote controls, and so on.
I never had the pleasure of designing with the 2901 bit-slice processor, but several engineers that I worked with over the years did. I did have the pleasure of working with the Fairchild F8 once upon a time.