Yes you can. There are many different ways of playing. Some people put a +4 card over a +2 card and some don't (I recommend to do it the +4 over +2 way)
No, in Uno, you cannot put a 4 card on top of a 2 card.
no way are you crazy
No, you cannot put a wild card on a Draw 4 card in Uno.
No, you cannot put a black card on top of another black card in Uno.
No, you cannot put a Skip card on top of a Draw 2 card in Uno.
No, you cannot put a Skip card on top of a Draw 4 card in Uno.
No, you cannot put a skip card on top of another skip card in Uno.
In Uno, you cannot put a Wild Card on top of another Wild Card.
Yes, in the game of Uno, you can say "Uno" before you put down your second-to-last card.
No, you cannot put a reverse on a Draw 2 card in Uno.
No, you cannot put a Draw 4 card on top of a Draw 2 card in Uno.
No, you cannot put a Draw 2 card on top of a Draw 4 card in Uno.