Sorry for chiming in when I have no idea, but here are the rules in wikipedia:
"If a Wild Draw Four card is played illegally (that is, if the player holds a matching color to one thatís on the DISCARD pile) and the person who plays it is challenged, the hand must first be shown to the player who has made the challenge. If the Wild Draw Four card has been played illegally, the offending player must draw 4 cards. If the card has been correctly played, the challenger must draw 2 cards in addition to the 4. The challenge can only be made by the player who is required to pick up the 4 cards after the Wild Draw Four card is laid."