In a 4-player game, when can players 3 and 4 interact with a showdown between players 1 and 2?

Ruling: Under current rules, other players can only play spells when the attacker or defender invites them, or when they become relevant (such as when a spell targets their units). However, the invite mechanic is being removed in updated rules, after which all players can play spells during any showdown like any other player.

Sequence: (Current rules with invites)

• Attacker or defender must invite other players when they have priority
• Invited player takes priority and must immediately play a card
• From then on, invited players are part of the showdown and can play/react as usual
• Players also become relevant automatically if a spell targets their units

Sequence: (Updated rules without invites)

• All players will always be relevant or the notion of relevant players will be removed entirely
• Whenever something is played, all players get an opportunity to play reactions in turn order
• During combat, all players get an opportunity to play actions in turn order

Nuances:

• The invite system had issues where it was possible to affect a third player from within combat without giving them a chance to react, which is why it's being removed
• The FAQ document explains the new rules for free-for-all multiplayer