Here's the official description of the seventeenth episode of Heroes Season 4 entitled "The Wall".
SYLAR'S WORST FEARS COME TO LIFE AS H.R.G. FACES HIS PAST. Fueled by his vision of Emma (guest star Deanne Bray) in grave danger, Peter (Milo Ventimiglia) attempts to rescue Sylar (Zachary Quinto) from his worst nightmare. At the carnival, Samuel (Robert Knepper) reveals H.R.G.s (Jack Coleman) secret past in order to bring Claire (Hayden Panettiere) to his side. Meanwhile, Lauren (guest star Elizabeth Rohm) makes a bold move to put a stop to Samuel. Sendhil Ramamurthy also stars. Ashley Crow, Jimmy Jean-Louis, Harry Perry, and Todd Stashwick also guest star.
Source : nbc.com