They probably picked the four most talked about contestants out of the eight at the time (this was before someone in the editing suite fell asleep on the bitch button for Melody's edit) and, as a function of the editing, because people who get right to the end tend to get more focus than people who dont, that's who has (probably) wound up in the Final Four.
I doubt they knew who was in the final four (they probably knew the next one or two firings) anyway and I don't know why they'd put them on the front if they did? They're not going to get retroactive extra sales once the public know who the finallists are, and it's not as though they can claim to be prescient like most people who pass spoilers off as spec are. They're just looking to shift units that week, so they put the most intriguing candidates on the front page.