What are the rules for stacking Draw 4 and Draw 2 cards in Uno?

In Uno, you can stack Draw 2 cards on top of each other, but you cannot stack Draw 4 cards. This means if a player plays a Draw 2 card, the next player can play another Draw 2 card to make the following player draw 4 cards. However, if a player plays a Draw 4 card, the next player cannot stack another Draw 4 card on top of it.