May you find the Perfect Shoe! xxx
Mostly offline, but trying.
I'm quoting from Wikipedia, but I believe this is the rules as printed:
When a Wild Draw Four card is played, the next player in sequence has the right to challenge the playing of the card if he/she believes that the player does have at least one card in the current color. The player of the Wild Draw Four card privately shows his/her cards to the challenging player. If the challenge is successful then the player who played the Wild Draw Four must draw four cards; otherwise, the challenging player must draw six cards and miss a turn. Regardless of the outcome, the Wild Draw Four remains on the discard pile and the color of further play is the color named by the player who played the card.