Sibel Kekilli plays Tyrion Lannister's lover Shae.
Samantha Bentley has yet to show up in an episode but is on the official cast list.
Jessica Jensen has yet to show up in an episode but is on the official cast list.
Aeryn Walker is playing one of Craster's wives in season four.
Maisie Dee was the prostitute Daisy in that scene with King Joffrey during that incredibly disturbing sex scene.
Sahara Knite appeared in sex scene with Esme Bianco back in season two.
For more on the specific roles they play, you should go to the site, but there might be some spoilers, so be warned.