A few words about the motivation.
The idea came from hellochess (
hellochess.com/ ). The problem with hellochess that it has ran out of players, so you cannot have a game of four player chess there now. In addition it had a timer related bug, so often your time just went to zero from tens of seconds or even minutes in an instant and you lost on time.
I adapted more or less the same board setup and rules. It is not enough to mate, you have to capture the opponent king. This make sense and adds additional tactical maneuvering ( you may lose material by capturing if the mate is supported by an other opponent piece for example, or it may be your interest to keep a side in play ). There is an unpredictible queening of pawns in endgames in the middle of the board at hellochess which I have not implemented. Since pawns are far apart en passant is also not implemented. Pieces of eliminated sides can still give check and support mate ( this may result in some very unexpected mates ). I have no chat, and in four player chess this is a plus, since this may give rise to organizing unhealthy alliances between players.
Four player chess has very interesting dynamics. Unlike standard chess there is no such thing as an equal exchange. An equal exchange between two sides is net material loss from the point of view of the two other sides. So you see an avoidance of captures in four player chess even when this seems very beneficial in conventional chess: even winning the exchange may be not a good idea for both sides will be down a piece compared to the other two sides.
King safety is much more important in four player because you can be checked by several players and while you avoid the check an other side may make use of this to capture one of your pieces in the same move.
At any rate I'm still looking forward to play four player games and would welcome any opponent.