[Jack_]

There's never been a sing-off with four acts left, it's more often than not been that case with five acts left, however people seem to forget there was a sing-off with five acts left in series 1 and 3, and instead choose to believe that last year's semi-final sing-off, where there were also five acts left, was a 'fix'.

Never ever has there been one with four left, and I reckon that'll be what'll happen in future now, with a sing-off still with five left.