Metal-poor stars observed with the Magellan Telescope.: II. Discovery of four stars with [Fe/H] <= -3,5 - Placco, Vinicius M; Frebel, Anna; Beers, Timothy C; Beers, Timothy C; Christlieb, Norbert; Lee, Young Sun; Kennedy, Catherine R; Rossi, Silvia Cristina Fernandes; Santucci, Rafael Miloni (The Astrophysical Journal, 2014)