Today's was the best round yet from an entertainment perspective, with three wins from five games. Hikaru Nakamura's win over Maxime Vachier-Lagrave makes him the clear leader with 3 out of 4, while Levon Aronian is the hero of the round after defeating Magnus Carlsen in a great game with sacrifices. Anish Giri also won, and quickly against Viswanathan Anand, while Wesley So and Fabiano Caruana should have defeated Sergey Karjakin and Vladimir Kramnik, respectively.
Nakamura - Vachier-Lagrave was a Najdorf, and White went for one of the unusual sidelines that has cropped in recent years, playing 6.Bd3 e5 7.Nde2. While that's unusual, the race between White's queenside expansion and Black's counterplay on the kingside is one sort of typical Najdorf middlegame. On this occasion Black's kingside play wasn't dangerous - at least when one defends as accurately as Nakamura did in this game. (Your mileage may vary.)
Aronian found an interesting new idea against the Semi-Slav in 10.Bc2, which is aimed against Black's ...e5-e4 ideas. After 24 minutes, Carlsen played 10...Rd8, and after spending 24 minutes on his next two moves, Aronian sacrificed the exchange and a pawn with 11.a3 Bxa3 12.Rxa3. After 12...Qxa3 13.c5 Black's queen is shut out of the game, both to its detriment and the rest of Black's army as well. This became evident when Aronian went for the Greek gift sacrifice 17.Bxh7+, resulting in a large advantage. Aronian's next dozen moves or so were the best ones, and while he made an inaccuracy on move 29 Black's position was extremely difficult to hold, and Carlsen failed to take advantage of his one chance.
The first two games ended before the first time control, and so did Giri-Anand. Anand has reputedly had some difficulties against the English in recent years, and he had some troubles in this game as well. Giri was outplaying Anand in the middlegame and had a won position until he chose 29.g5 rather than 29.Rh5. The error was more than compensated by an even bigger mistake by Anand on move 31. The former champion had to play 31...Qxh4, giving up a piece but getting enough pawns and positional compensation to save the game. Instead, 31...Nc5 lost on the spot: 32.g6 Qd7 33.Bb4, and Black has no good defense against d4.
As for the draws, So was crushing Karjakin until his careless 34.Qxc4??; instead, any move defending the rook (e.g. 34.Re2) would have won easily. The problem was that 34.Qxc4 allowed 34...Nf6, giving Black enough activity to survive. After this both sides played great chess, with So setting Karjakin a series of very difficult problems to solve, and Karjakin rose to the occasion every time. Caruana too was winning against Kramnik, and from early on. Kramnik's 15th and 16th moves were errors, but after that he went into Tal mode, blew a thick fog over the board, and Caruana couldn't manage to put him away.
The games are here, with annotations to Aronian-Carlsen. Here's what's on tap for round 5: