Gotham Knights was announced with a 2-player Koop mode, but now it seems that he has been expanded to 4 players.
This information comes from the PlayStation Network, which has made a 4-player co-op mode with an update to Gotham Knights from the 2-player co-op description.
It remains to be seen whether there is really something or whether the error devil may have crept in with the Gotham Knights Update.
An official announcement or a Dementi is still pending.