If theoretically, I passed 4 relay candidates to an agent, will the agent determine the priority of which candidate to use based on latency/RTT?
I've read that in some messages on SO, but have been unable to find that in the actual WebRTC codebase. If that is the case, is it only for some implementations of WebRTC?
No, the priorities will be used. https://datatracker.ietf.org/doc/html/draft-uberti-mmusic-nombis-00 describes a better approach but its not implemented.
https://datatracker.ietf.org/doc/html/draft-thatcher-ice-renomination-00 is implemented and can be activated through a parameter enable_ice_renomination in the configuration but it isn't documented further.