Entertainment Weekly has the official Styr casting news, which comes after months of speculation about who will play the character. Gatt seemed to fit the bill, so now it’s unclear who he will actually be playing in Season 4. Kolokolnikov is a well-known Russian actor, though he hasn’t had much screen time in the US.
Styr is a wildling leader of the Thenn who allies himself with Mance Rayder to attack the Wall. In the novels he is among the wildlings sent ahead to attack the Wall who Jon Snow betrays to return to the Night’s Watch, but it seems like he’ll be introduced instead during Mance’s onslaught on Westeros.
Kolokolnikov’s casting seems to be the last of Season 4, which is currently filming in Europe. The new season will premiere on HBO on Spring 2014.