Aang, the Last Airbender

Whenever Aang enters the battlefield or attacks,
put a +1/+1 counter on it and choose a color.
Then reveal cards from the top of your library
until you reveal a card of the chosen color and
put it into your hand. Shuffle all other cards
revealed this way into your library.
Exile four cards with different colors from your
hand: Transform Aang. Activate this ability only
if Aang has four or more +1/+1 counters.