The term Fourth Estate refers to the press, both in its explicit capacity of advocacy and in its implicit ability to frame political issues. The term goes back at least to Thomas Carlyle in the first half of the 19th century.
Novelist Jeffrey Archer in his work The Fourth Estate made the observation: "In May 1789, Louis XVI summoned to Versailles a full meeting of the 'Estates General'. The First Estate consisted of three hundred clergy. The Second Estate, three hundred nobles. The Third Estate, six hundred commoners. Some years later, after the French Revolution, Edmund Burke, looking up at the Press Gallery of the House of Commons, said, 'Yonder sits the Fourth Estate, and they are more important than them all.'"
. . . The affairs of the nation were there deliberated and decided; what we were to do as a nation. But does not, though the name Parliament subsists, the parliamentary debate go on now, everywhere and at all times, in a far more comprehensive way, out of Parliament altogether? Burke said there were Three Estates in Parliament; but, in the Reporters' Gallery yonder, there sat a Fourth Estate more important far than they all. It is not a figure of speech, or a witty saying; it is a literal fact,--very momentous to us in these times. Literature is our Parliament too. Printing, which comes necessarily out of Writing, I say often, is equivalent to Democracy: invent Writing, Democracy is inevitable. Writing brings Printing; brings universal everyday extempore Printing, as we see at present. Whoever can speak, speaking now to the whole nation, becomes a power, a branch of government, with inalienable weight in law-making, in all acts of authority. It matters not what rank he has, what revenues or garnitures. The requisite thing is, that he have a tongue which others will listen to; this and nothing more is requisite. The nation is governed by all that has tongue in the nation: Democracy is virtually there. Add only, that whatsoever power exists will have itself, by and by, organized; working secretly under bandages, obscurations, obstructions, it will never rest till it get to work free, unencumbered, visible to all. Democracy virtually extant will insist on becoming palpably extant. . . . (Italics added)
This was not Carlyle's first use of the term. If, indeed, Burke did make the statement Carlyle attributes to him, Burke's remark may have been in the back of Carlyle's mind when he wrote in his French Revolution (1837), "A Fourth Estate, of Able butts, springs up. In this context, the other three estates are those of the French States-General: the church, the nobility and the commoners, although in practice the latter were usually represented by the middle class bourgeoisie.
An early citation for this use—earlier than for the one that now prevails—is Henry Fielding in Covent Garden Journal (1752):
None of our political writers ... take[s] notice of any more than three estates, namely, Kings, Lords, and Commons ... passing by in silence that very large and powerful body which form the fourth estate in this community ... The Mob.
Fourth Estate has referred to "the public press" since at least as far back as the early 1800s. More generally, it has also been used to refer to any group other than the clergy, nobility, or commons that wields political power.
The role of news gathering organizations in society as a check on the abuses of the powerful, and a provider of information needed to keep citizens informed, thus supporting public discourse. -Brian Feeney