Hello! I've uploaded a new build named for-the-queen-windows.zip which has support for 4 players using the same keyboard. I haven't tested it with 4 players myself but at least with 2 players seems to work fine. Below are the keyboard controls for each player:
P1
Move: W, A, S, D
Attack: C
P2
Move: I, J, K, L
Attack: N
P3
Move: Up arrow, Left arrow, Down arrow, Right arrow
Attack: Enter
P4:
Move: Numpad 8, Numpad 4, Numpad 5, Numpad 6
Attack: Numpad 0