That's kinda the whole point of PeerTube: the people watching your video also become peers in the WebTorrent swarm so no one party ends up eating 100% of the bandwidth cost. To use your example: if "if a million people want to download my videos in 4K" then you will have a LOT of peers in that swarm. The more peers in a swarm the less likely _any_ PeerTube instance will have to serve _any_ video content at all.

TLDR; popular videos are the most likely to have little-to-no bandwidth cost for the content creator/instance admins.