Collapsed Stadium.

Each player can't have more than 4 Benched Pokémon. If a player has 5 or more Benched Pokémon, they discard Benched Pokémon until they have 4 Pokémon on the Bench. The player who played this card discards first. If more than one effect changes the number of Benched Pokémon allowed, use the smaller number.

This Stadium stays in play when you play it. Discard it if another Stadium comes into play. If a Stadium with the same name is in play, you can't play this card.