It was too good to last. After 19 consecutive draws, Fabiano Caruana ruined everything by winning the last game of the round, with Black, against Sergey Karjakin. He showed some great preparation (though Jan-Krzysztof Duda gets the finder's fee for being the first to play 8...b5) and played an all-around excellent game. Unfortunately, while Karjakin showed the right spirit yesterday by accepting Levon Aronian's draw offer, despite having a winning advantage, Caruana didn't repay the favor. May he be punished with many more wins in the rest of the tournament.
Everyone else deserves to be commended for doing their part. Aronian played a quasi-Marshall Gambit against Viswanathan Anand, and Anand immediately fled from danger by returning the pawn to create a drawish position. Hikaru Nakamura may have had a slight edge against Ian Nepomniachtchi, but the latter defended well (he was particularly pleased with his 11...Re8). Nakamura's 23.f4 dissipated his advantage (it was a slightly weakening move that should have been kept in abeyance), and Nepo's fortress held. Wesley So played a reversed Benko Gambit against Michael Adams, and while he eventually managed to equalize (with White) he got no further.
While the three games mentioned in the last paragraph were all drawn in around 30 moves, Maxime Vachier-Lagrave and Magnus Carlsen was less balanced. MVL pressed on the white side of a Giuoco Piano for a long time, and Carlsen's pawn sac to obtain the bishop pair wasn't working out for a while. The key moments were moves 28 and 29. White had to find some way of organizing his knights to give them stable squares and to neutralize Black's bishops. It looks like 28.Nfd4 was the best way to achieve that, but Vachier-Lagrave's 28.Nb4 followed by 29.Nd3 didn't do the job. In the post-game interview he said that he had seen the position after 33.Nc5 Bc8, which arose naturally if not by force after 28.Nb4. He assumed that he'd have something there, but once it arose he realized that there was nothing, and at that point it was time to pull the plug and ensure the draw.
[Side question about Carlsen. In the post-game interviews with Magnus Carlsen it's hard to tell whether he's unhappy about the draws or about having to interact with Maurice Ashley. Carlsen is generally a pretty lousy interviewee unless he has just won or things are going well, but he seems to be expressing disgust, even revulsion in his self-presentation. I remember he and Ashley had a very uncomfortable interview at one point - I think back in 2016 - but I also thought they had gotten past that. Magnus-watchers, what do you think? Whatever it is, it would be good for him to learn to present himself in a more human way: Ashley is just doing his job, and he's a rich man because of chess fans, not in spite of them.]
The games will come later; for now, here are the tomorrow's pairings for round 5:
Nepomniachtchi and Anand haven't played yet, so it's not too late to fix it: Karjakin can beat Nepo, Anand can beat Caruana, and then in round 7 Nepo can beat Anand. The tournament's ultimate perfection has been spoiled, but that's the next best thing.
UPDATE: The games are here, with some comments to Vachier-Lagrave vs. Carlsen and Karjakin-Caruana.