There should also be a +4 stacking, as well as the +4 challenge which depends on the card played before the +4 card. If the player who played the +4 card doesnt have any cards of the same color or number as the card before the +4 card, the one who challenges that player will fail the challenge and draw 6 cards. however, if the player who played the +4 card has any cards of the same color or number as card before +4 card, the one who challenges the player will win the challenge and the one who played the +4 card will instead draw 4 cards.
There should also be a +4 stacking, as well as the +4 challenge which depends on the card played before the +4 card. If the player who played the +4 card doesnt have any cards of the same color or number as the card before the +4 card, the one who challenges that player will fail the challenge and draw 6 cards. however, if the player who played the +4 card has any cards of the same color or number as card before +4 card, the one who challenges the player will win the challenge and the one who played the +4 card will instead draw 4 cards.
I hope there will be a +4 challenge feature.