Pokémon Pokopia allows up to three other players to join a multiplayer session.
That means a session can include the main player plus three visitors. Multiplayer activities let players visit and spend time together in the world they are shaping.
Nintendo's launch information describes support for both local wireless and online play. Each visiting player can also bring a Pokémon along, making the feature fit the game's cooperative and social design.
The number refers to other players, not the total number of people present. Counting the host, the maximum session size is four players. This distinction explains why descriptions may mention either three visitors or four players overall.