The biggest drama on television returns in two weeks' time for its eagerly awaited fourth season, bringing a host of questions. Will Jon Snow hold back the forces of darkness converging on The Wall? Can Daenerys Targaryen control her fast-growing dragons and cross the narrow sea to claim the iron throne of Westeros? Will Tyrion Lannister continue to astutely navigate the political corridors of Westeros's capital, King's Landing, or does the return of his Machiavellian father Tywin mean trouble for everyone's favourite sharp-tongued dwarf?

If the above paragraph reads like so much nonsense, then chances are you're one of the few people who haven't jumped on the growing Game of Thrones bandwagon. In four years, this epic (and epically convoluted) tale of a kingdom torn apart by machinations, murder and mayhem has become the most talked about show on TV, critically acclaimed, widely watched and referenced on everything from South Park to Parks and Recreation.

So why has this show come to exert such a grip on our imaginations? Adapted by David Benioff and D B Weiss from George R R Martin's best-selling books, Game of Thrones is a dark and bloody fantasy drama set in a quasi-medieval world and featuring a huge cast of characters, many of whom are prone to delivering doom-laden speeches about the corrupting nature of power. It's sprawling, complicated, and occasionally cheesy, and it really shouldn't work as effectively it does, dragging the audience along for a brutal but exhilarating ride that all too frequently ends with viewers saying, "I can't believe they just did that", as yet another fan favourite ends up face down, dead in the dust.

Yet it does work. Game of Thrones is HBO's biggest hit since The Sopranos, averaging more than 13 million viewers per episode, as well as being the world's most pirated show, with the finale of season three illegally downloaded six million times. In the UK it pulls in over a million viewers per episode for Sky Atlantic, while in February, season three became the fastest selling TV box set in a decade. At a preview screening at the Barclays Centre in Brooklyn, New York, fans queued for hours, dressed as their favourite character, and last week there was talk of movie spinoffs, with Martin telling the Hollywood Reporter: "It might need a feature to tie things up."

There are cats and dogs called Tyrion, after the schemer played by Peter Dinklage, and a growing generation of girls named Arya and Sansa in honour of the long-suffering Stark sisters. YouTube is full of inventive fan videos and you can purchase anything from handcrafted dragons' eggs to a replica Iron Throne.

Celebrity fans include Barack Obama, who recently joked that he'd asked HBO to get him DVDs of the new season, and Facebook CEO Sheryl Sandberg, who said she identified with Daenerys Targaryen, despite the fact that Daenerys is more likely to incinerate the competition than ask them to lean in.

"Game of Thrones sits at a sweet spot between fun and serious," says Alyssa Rosenberg, culture writer for the Washington Post. "It has dragons! Sword fights! But it's also extremely clear-eyed about politics, gender and sexuality, and the vicious inequalities that produce some of its fun. That makes it highly enjoyable, but also gives the show a more substantive claim to the kind of political insight that so many prestige dramas claim."

It wasn't always like this. When the show began in 2011, and despite the huge fanbase in place, everyone seemed desperate to play down its fantasy roots.

During publicity tours, the talk was all of "The Sopranos meets Middle-earth", with the emphasis firmly on the dark deeds of David Chase's mob drama rather than JRR Tolkien's epic tale. Parallels were drawn with historical events and newspaper articles compared it to everything from Greek tragedy to Dynasty. The subtext: don't mention the fantasy. Despite the success of Peter Jackson's The Lord of the Rings trilogy, there was a whiff of snobbery.

"It might work on a big screen where audiences are looking for escapism," said the naysayers. "But you'll never build an audience on TV. People will be turned off by the swords and the sorcery and, most of all, by the names." That prejudice was reflected in some sniffy early reviews, most notably in the New York Times, which dismissed the show as "boy fiction patronisingly turned out to reach the population's other half", before wearily concluding: "If you're not averse to the Dungeons & Dragons aesthetic, the series might be worth the effort."

In Britain, Caitlin Moran penned a witty mea culpa last year, retracting her negative review of the first season, claiming: "I made a mistake, I didn't get it. I didn't see." She was not alone. Even those of us who had devoured the books were wary of how to sell the adaptation. "Yes, it's got magic in it, but it's not elves and quests, it's really about politics and betrayal," ran one of my favourite gambits. "Yes, the book covers do have lurid pictures of half-naked women on them, but that doesn't really capture the story or the dark appeal of Martin's world …"

Those lurid covers are long gone. Gone, too, is the sense that fantasy is a dirty word – Da Vinci's Demons, Black Sails and Outlander are all trying to capture a similar mix of epic sweep and dark deeds. "There's a light yearning for fantasy in pop culture," says Gawker writer Michelle Dean. "It has nothing to do with the genre, so much as longing for something escapist."

It also helps that it is the best sort of popcorn television, an entertaining mix of political skulduggery, big-budget battles and, of course, nudity. "The nudity certainly helped it become water-cooler television," says Dean of the many scenes in which characters just happen to be naked when discussing major plot points, a device one US critic labelled "sexposition".

Yet the nudity for nudity's sake has diminished. As Daniel Mendelsohn argued in the New York Review of Books: "Those who complained about the TV series' graphic and 'exploitative' use of women's bodies are missing the godswood for the weirwood trees: whatever the prurient thrills they provide the audience, these demeaning scenes … also function as a constant reminder of what the main female characters are escaping from."

Thus Lena Headey's Cersei is at her most sympathetic when railing against her marital fate, while the joy of Daenerys is that she has an advantage most of the female characters do not: the three baby dragons who bolster her claim. "I don't know how many hardened fantasy-haters can hold out against baby dragons for long," jokes Rosenberg, adding: "Game of Thrones gets a pass from a lot of folks because of its brutal politics and the sense that it's not very interested in happy endings."

But the big question is, will there be an ending at all? Martin has written five books out of a proposed seven-book series, with book six due in 2015. But this is an expensive show and HBO is committed to seven seasons, which would mean a finale in 2016. It seems inevitable that a compromise will have to be made – with Benioff and Weiss deviating from the books or sticking with Martin's proposed ending and spoiling the novels for those who have patiently waited for the story to unfold.

Both sides remain upbeat that a satisfactory conclusion will be reached. Until then, the focus remains on the start of season four. As spooky red priestess Melisandre has repeatedly promised: "The night is dark and full of terrors." In a fortnight's time, millions of eager viewers will tune in to see what shape those terrors take.

Game of Thrones season four starts on Sky Atlantic on 7 April at 9pm