I assume you're talking about when somebody plays a Wild Draw 4 card against you.
Here's how it is:
- If your opponent plays a Wild Draw 4 card and you don't challenge, you draw 4 cards - If your opponent plays a Wild Draw 4 card and you challenge, and your opponent could've played another card, your opponent draws 4 cards - If you opponent plays a Wild Draw 4 card and you challenge, but your opponent couldn't've played another card, you draw 6 cards
As a for instance, you should never challenge a Wild Draw 4 card when your opponent has just drawn from the deck, because obviously he could not have played a card and must have just drawn that one.
Lady Nancy Astor: Winston, if you were my husband, I'd poison your tea. Sir Winston Churchill: Nancy, if I were your husband, I'd drink it!