I know nothing and am not too bright so be kind.
I see the 200-400 as part of the 24-70/70-200 f/2.8 L II line of lenses.
I expect the 300-600 will be part fo the 24-70 f/4 L , 70-300 f/4-5.6 L line of lenses.
Relative to the 200-400 I'd expect to sacrifice some image and build quality but in return get significant $ in my pocket and a relatively lighter lens. I'm going to set my flag at the $8000 price point. I would not be surprised if it was even lower.
Like I said, I know nothing and am not too bright so be kind.