Not the "same" image but close enough. Perpective, compression, dof, etc... Will change from 400 to 600mm.
The big difference will be 21 to 47 mp.
Paco, I guess it was a hypothetical question about physical conditions ... not a question about artistic aspects ... For me the main point is that I tend to leave the expensive stuff at home because I hate carrying it with me if it is too heavy.
In that sense, the will be probably NO PICTURE, zero pixels for me the D850 and a 4/600 lens. BUT there are already thousands of wonderful pictures taken with my D500 TC14v3 and the 4/300PF lens.
300mm*1.4 = 420mm @997g, delivers good enough images for me. 21 Megapixels on my D500 at an angle of view of a 630mm lens, or 600mm at 3810g. One Stop better and 47MP on my D850.
"Boy, you gonna carry that weight a long time" The Beatles