I was watching the Chess24 stream with Peter Svidler as commentator, and he discovered the mating attack in game 4 a few minutes before it was played. As both Carlsen and Karjakin played the expected moves leading up to Rc8+, there was a lot of tension about whether Carlsen had seen it and whether he would dare play it (I saw a later analysis saying that if black plays ... Bf8 it's mate in something like eight moves, so that's some calculation even if it's fairly straightforward).
Edit: 49. Rc8+ Bf8 50. Rxf8+ Kxf8 51. Rxf7+ Ke8 52. Rf8+ Kd7 53. Qf7+ Kc6 54. Rc8+ Kb5 55. Qc4+ Ka5 56. Ra8#