@Odoritomaki 4000 cal as bodybuilding diet just shows that it's a comparatively lazy sport. XD
Elite cross-country skiiers need 8000 calories per day, and there seems to be a general consensus among them that eating is the dull part (and what stops them from getting more training done.
(I found an English source saying much the same, but with slighly lower numbers. Our Swedish star usually ran by 8500 calories, and he definitely said "I'd train more if I could, but I just can't eat more" https://www.nytimes.com/2018/02/23/sports/olympics/cross-country-skiing-food.html )
Those bigger numbers you cited.. well, they seem weird (or fattening).