- Fixed an issue where connecting 4 controllers to Steam Deck would not work properly. Now, it is possible to connect and use 4 external controllers by navigating to "Help & Options" > "Controllers" > "Assign", and assigning the 4 external controllers (making sure the Steam Deck itself is not assigned to any of the slots).
Support 4 controllers on Steam Deck
Source: Steam Community (opens in a new tab)