Perhaps this:

"The Fourth Amendment (Amendment IV) to the United States Constitution is the part of the Bill of Rights that prohibits unreasonable searches and seizures and requires any warrant to be judicially sanctioned and supported by probable cause."

Possibly to the Fourth Estate? It would have been clear to Snowden that the clergy and commoners aren't doing enough to control our kings and princes.

Fourth Estate seems very plausible. The idea of the 'Fourth Estate' is that the media (the fourth estate) keeps an eye on the government. Whether this is the intention or the 4th Amendment is the intention, the other of the two is a nice coincidence.

There is also a Fourth Citizen in Shakespeare's "Julius Caesar": http://www.jstor.org/stable/802843

I don't see the connection, though.

Ellsberg, Felt, Manning, Snowden?

I wondered whether this paid homage to other notable leakers before him.

When written as one word, "Citizenfour" as in the movie's title (not sure if he actually used it like that), my first thought was possibly a connection to "carrefour," which is French for a road intersection. The 4th amendment theory seems more plausible, but a crossroads provides some interesting imagery in ralation to Snowden's intentions vs the current course of mass surveillance.

The exact wording he used was "Citizen Four", two words.

I read it as "Citizen 4". Then, if you reverse the order ("4 Citizen"), it becomes close to "for the citizens".