Could coyotes kill a 350 or 400 lb calf?

If they had enough in a pack to bring one down, and were hungry enough, yes. Coyotes that are alone won't bring down a calf themselves. But if there are more than one, like 2 or 4 in a group, then they could definitely bring down a calf. Most cases of calf killings are not by coyotes, but by feral dogs or wolves.