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, G16 | EF: SY14/2.8, V20/3.5, 24/2.8, 35/2, Ʃ35/1.4, 50/1.8, Ʃ50/1.4 EX, 100L Macro, 17-40L, 24-105L, 70-200/2.8L IS II, 1.4x II, 2.0x III, 70-300L, T28-300 Di VC PZD | E: SY12/2, 35/1.8 OSS, 16-70 ZA OSS, 55-210 OSS, SY300/6.3 ED UMC CS, Metabones SB | Vintage FD & FL glass.