The "Big Four" are the four teams Manchester United, Arsenal, Chelsea and Liverpool, who have made the Premiership one of the most utterly predictable leagues in Europe over the past few years.   Everyone knows at the start of the season that the Big Four will take the top four places at the end of the season, with the only slight intrigue being in which order they finish, and who will come "best of the rest" in fifth. This season, for the first time in years, there's an outside chance it might change, due to at least one of the four having a laughably poor season.

The Big Four were invented because the FA had become irritated by smaller, poorer teams challenging for the league championship in the 1980s.  Although they were prepared to tolerate the occasional surge from teams like Aston Villa and Everton, the FA's patience snapped when backwater nobodies Norwich mounted a credible title challenges for the best part of the 1988/89 season.  The FA recognised that people who only watch football on telly wanted to see easily recognisable teams playing in red or blue, and thus concocted a plan to make it so.
The FA realised that drastic action was required in order to freeze out these over-achieving minnows, and together with Sky came up with the Premiership in 1992.  All previous football records were wiped from history, and the FA appointed Liverpool, Arsenal and Manchester United to divvy up the title race between the three of them for the next decade.
The only real hitch came in 1995 when Blackburn Rovers somehow managed to win the title, due partly to Manchester United and comedy striker Andy Cole managing to blow a golden chance to snatch it on the last day by drawing at West Ham.  This proved to be a mere blip however, and Blackburn remain the only team to have won the title other than Manchester United, Chelsea and Arsenal since the Premiership began in 1992.  And they won't win it again, that's for sure.  Even Liverpool haven't won the title, but they are permitted to cling on to their Big Four status anyway, despite never having mounted a sustained title challenge in twenty years.
However, the FA noticed that they still didn't have a team playing in blue in this plutocracy, which was confusing armchair fans no end.  The vacancy had originally been earmarked for longhouten rangers, who had been considered part of the "Big Five" in the 1980s (a totally different concept to the Big Four), but who had unfortunately proved to be inconveniently and utterly shit during the following decade. 
Instead, lovable businessmen / outspoken character / graceless old sod (delete as appropriate) Ken Bates suggested that his Chelsea team step into the void to make up the big four.  The FA were unsure at first, but the arrival of stubbly-faced Russian billionaire Roman Abramovich at Stamford Bridge clinched it, as Abramovich was able to shit all over the wealth of the other big three combined.

Maintaining the Big Four

Cunningly, the FA made sure that the Big Four would receive bagfuls of cash from their TV deals to ensure that they kept the title race amongst themselves.   This plan really took off with the development of the Champions League,  guaranteeing that the top three or four teams in the Premiership would earn far more cash than any other teams in the league could dream of, and ensuring that their qualification for the following season's Champions League would thus become a self-fulfilling prophecy.
Another popular device used by the Big Four is to only buy their players from abroad, thus preventing any cash from trickling down to any other teams in the English leagues.  This has proved to be a stroke of genius, as the other English teams have therefore inflated the prices of their players to make up for the lack of transfer income, allowing the Big Four to justify shopping abroad instead.  
At the same time, the Big Four have been allowed to poach youngsters from the academies of the smaller English clubs for laughable pittances, ensuring that all of the best young English talent can rot for years on end in their reserves.

Challengers to the Big Four
The FA have until now managed to crush the ambitions of every other Premiership club to the extent where coming fourth in the league table would be beyond their wildest dreams.  The thought of anyone else actually winning the title is ludicrously laughable.  Everton are the only club to have managed to separate the Big Four since 2003, when they came fourth in 2005.  The governing bodies recognised the harm that having another team in blue in the mix might do to their plans, and promptly introduced a special Champions League place for Liverpool, who had finished fifth.  Normal order was resumed the following season.
Aston Villa looked like they might have a crack at the top four places for most of the 2008-09 season.  However, Villa slumped spectacularly to finish sixth, chucking away that season's UEFA Cup run for good measure.

In the 2009-10 season Tottenham took fourth place from Liverpool, whose plan of having two good players and a rotating roster of nine terrible ones for the rest of the season backfired when their two good players got injured. As punishment they were ordered to take an English manager for the next season and their fourth spot is currently under review as Manchester City compete with Harry Redknapp as to who can offer the biggest bribe to the FA.
Manchester City are the latest club to gash away hundreds of millions of pounds in the hope of finishing one place higher than fifth.  However, no-one is taking them seriously just yet, given City's ability to cock up anything which looks remotely like a promising position.

Community content is available under CC-BY-SA unless otherwise noted.