Short answer: find the "4PC" button and click it.
Long answer: open "custom" bar -> custom variants, then scroll down until you find the "4pc" button and click it, you get "W&B". Then, click "show more", find a bar of "is dead"(or anything named like that), let blue and green dead by making it grey.
I made a chess variant on a 14x14 board, but It says it’s a 4 player game, but I only have two piece colors on the board. White and black. Please help.