Can you place a draw 2 (+2) on top of a draw 4 (+4) in regular Uno?
You can as long as it's your turn and the color of your Draw 2 matches the color specified by the player who played the Draw 4. In standard Uno rules, the player after the one who played the Draw 4 will draw the 4 cards, and the player after that one gets the next turn. So, you would need to be the player next to the player next to the one who played the Draw 4 to play your Draw 2 (again, assuming it's the correct color). The player after you would draw 2 cards, and it would be the next player's turn.

An example from @JoeW: player 1 plays a Draw 4 and calls red, player 2 draws 4 cards, player 3 plays a red Draw 2, player 4 draws 2 card and play continues as normal.