This was an issue at our last gaming session: how many board pieces do you use for 4 players?
It seems logical to use one board piece per player, although I believe that this is not the intent, and I was unable to convince my opponents otherwise.
Not sure why they wouldn't believe you, the setup rules explicitly tell you that each player in turn order adds one tile to build the board, and sets out the conditions for doing so.
"The first player draws a landscape tile and places it in the middle of the table. The remaining players follow in player order and draw landscape tiles and place them as a game board. The second tile must be adjacent to at least one edge of the first tile. Each following landscape tile must be adjacent to at least two other landscape tiles."