Icoras, Epitome of Knowledge

Flash
When "Icoras, Epitome of Knowledge" enters the battlefield or attacks, you may look at the top 4 cards of your library. Reveal an instant from among them and place that card on the top of your library. Put the rest on the bottom of your library in any order.
Whenever you cast an instant from your hand, you may reveal the top card of your library. If it is an instant with the same converted mana cost, you may cast it without paying its mana cost.