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The fourth series of the BBC’s smash-hit competition series has reached the point at which successful reality shows must decide what to do. Should it stick to the tried-and-tested format, which has proven to be astonishingly appealing, or should it adapt and evolve, in an attempt to fend off the fear of repetition? For The Traitors, the question is particularly pronounced. The overall premise — strangers gather around a table and try to work out who is lying and who is telling the truth — is a simple parlour game. There is always a danger that it might grow tired, particularly when it doesn’t have the star power of celebrity contestants to fall back on.

Tweaking it slightly is the smart move, and it is clearly working. The fourth series introduced the biggest twist yet, of the red-cloaked secret traitor, whose identity was kept from contestants and viewers, allowing audiences at home to play along for a while. Not knowing who the final traitor was livened up the round table discussions for the viewer. It is all extra-paranoid and tetchy, and deliciously so. The gothic-tinged sports day-esque games, which build the funds in the prize pot, are even more fun and devious than they have been in previous series. Who knew that re-enacting old paintings could be so entertaining?

But ultimately, it is down to the casting, which continues to be exquisite. From the beginning, The Traitors felt like an outlier to other reality shows due to its “ordinary” contestants: it had players of different ages, from many backgrounds, and most of them seemed less polished and camera-ready than your average fame-seeking recruit. In a sign of its continuing evolution, this year only one contestant claimed that they wanted to be a faithful, while all the others insisted they would make good traitors — a marked shift towards the dark side. There may be an argument that this year’s group has skewed slightly younger, and that they are all more informed as to how to play the game, but the game remains gripping because it refuses to oblige even the best-laid plans.

The contestants play gothic-tinged sports day-esque games in order to add money to the prize pot

The final episode of The Celebrity Traitors brought in a massive 15mn or so viewers, making it the most-watched broadcast on British television last year. Viewing figures for the ordinary version, while growing, have been nowhere near those heights; the series three finale brought in an impressive but less astounding 7.4mn. In its fourth series, The Traitors suggests that normal people have the edge, if not yet the audience. The stakes are higher, the deceptions are greater — celebrity status makes it harder to lie about your profession, after all — and the potential for bad behaviour is always on the surface. The show continues to reach new heights.

★★★★☆

On BBC1 and BBC iPlayer on Wednesday to Friday at 8pm until January 23