There is no way it is $4000 better than the same Nikon 600mm f/4 in computer designed lens era. Canon is just being greedy because they know there are fools who will buy this lens that don't really need it just to show off.
What "same Nikon" lens? Nikon's 600 f/4 tops the scales at eleven and a quarter pounds; Canon's a mere eight and a half. Nikon isn't even pretending to play in the same league, let alone ballpark.
Hell, Canon's 600 f/4 is significantly lighter than even Nikon's 400 f/2.8. Your entire Canon kit, big gun, bodies, 70-200, and the rest may well weigh less than just Nikon's lens.
Both of these companies are making fantastic pieces of optical imaging equipment, but only one is in the business of making photographic gear for photographers.
Yeah, lighter doesn't mean better. How are they saving that weight, by replacing metal for plastic? I'd rather have a heavier sturdier lens made of metal than something with increased plastic. A t3i sure is light but I am not using one over a 5dmark 3.