I don't see them holding Tywin's death until the end of the season. As 137th Gebirg
says, the big event is usually in episode nine, with the finale doing smaller shock reveals that set up new arcs (Lysa's death, maybe Tyrion's meeting with Illyrio if they don't want to leave his destination a mystery). The Oberyn/Mountain fight would have to come earlier than episode nine and isn't really an "event" so much as a set piece featuring secondary characters. The timing of the Wall story is trickier to work out, but I imagine Stannis will have arrived before episode nine anyway, since before the end of the season they need to play out Stannis' offer to legitimize Jon, the debate over the new Lord Commander, and Jon's eventual election.
I always figured they might do all the Bran chapters from Storm
in season three. Season four can stretch the two beyond-the-Wall chapters from early in Dance
, ending with his reaching Bloodraven, and then season five can be all about the single chapter where he's in training, which actually covers a lot of time and significant narrative ground.