A cool, steady stream of anxiety, Laura Poitras' documentary "Citizenfour" draws from the visual language and buggy paranoia of the best-known 1970s political thrillers: "The Conversation," "The Parallax View," "Three Days of the Condor," "All the President's Men." Each of the cities filmed in "Citizenfour" gets its own quiet yet sinister establishing shot, so that Rio de Janeiro, Hong Kong and London each look like twinkly beehives of undisclosed activity.