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.
5D3, 5D2, Sony α6000 | SY14 f/2.8, Ʃ20 f/1.8, 24 f/2.8, 35 f/2, Ʃ35 f/1.4A, 50 f/1.8 I, Ʃ50 f/1.4 EX, 100L Macro, 17-40L, 24-105L, 70-200 f/2.8L IS II, 1.4x II, 70-300L, 100-400L | E-mount: SY12 f/2, Ʃ19 & 30 f/2.8 EX DN, 16-70 ZA OSS, 55-210 OSS, Metabones SB | FT-QL, AE-1P | FD(n) & FL lenses.