When a player decides to play a Wild Draw Four Card, also know as a +4 Card, that get to choose the color that continues play PLUS the next player must draw 4 cards from the DRAW pile and lose their turn. However, there is a hitch! A player may only play this card when set player does NOT have another card in your hand that matches the COLOUR on the DISCARD pile (but it is acceptable to play this card if you have matching numbers or Action Cards). If turned up at the beginning of play, return this card to the deck and pick another card. Much like the stacking of +2 cards, many players have decided to ignore this rule as it makes the game generally less fun.
If a player suspect that a Wild Draw Four card has been played on them illegally (i.e. the player has a matching card), then the suspecting player may challenge that supposedly cheating player. The challenged player must show the challenger their hand. If guilty, the challenged player must draw the 4 cards instead of the original receiver of the +4 card. However, if the challenged player is innocent, the challenger must draw the 4 cards PLUS an additional 2 cards (6 total)!
If a player currently has one of these cards in their hand while another player manages to win the round, the winning player is awarded an extra 50 points.