Board Game Arena

When searching for a game, I can select 3 or 4 players, but I can't find how to make it flexible. Please help.

I think you can't. But you may open or search for one game with 3 and one with 4 players at the same time.

If you are using the automatic lobby (Play now -> Chang from "Friends" or "Manual" to "Automatic") you have a cog symbol at every game where you can specify your preferred settings. You can choose 3 players AND 4 players.