I'd love a 600/4 with a built-in 1.4x TC. Not sure we'll see one soon, though.
Been wishing for this since the 200-400 w built-in 1.4 was announced. Actually, I think there is a good rationale for building a 1.4 into all the superteles above 400mm. The added weight/cost will not outweigh (puny) the extra usability for the intended consumers. I know I often switch the 1.4 on and off my 600 when stalking wildlife and it requires too much fiddling around - even if you get 'good' at it. I'm less enamored of the big zooms just b/c I need the speed and reach of the primes in 99% of my photo situations. Having a built-in 1.4 gives you that little bit of quick flexibility to frame the shot better. And quickly switch from say a whitetail at 40 yards (600) to a chickadee at 15 yards (840).