There were three decisive games today, and there are three players advancing to round 5, but there isn't a one-to-one correlation between the two "threes". Ding Liren defeated Wang Hao in a good game with White in a Catalan, but if Wang Hao had known about an earlier game - or simply found the right idea on move 22 - the game probably would have finished in a draw, and they'd be off to tomorrow's tiebreaks.
Levon Aronian also won, defeating Daniil Dubov in a long game. Aronian reached a theoretically won ending, and while he had time at the start to figure out how to win it, he didn't hit on the right plan. Over the course of the next many moves, he even allowed Dubov numerous chances to draw, but Dubov - who had the time and ability to work out his drawing opportunities - thought it was the better strategy to keep blitzing Aronian. It backfired. It took Aronian seemingly forever, but around 40 moves later than he could have won, he finally hit on the right strategy - though he still managed to give Dubov one more (missed) drawing chance after that. Should Dubov have taken his time? The problem is that if he did, at a moment when he didn't have a draw, it could very well have given Aronian the chance to work out the winning plan. So I think Dubov was generally right to blitz - given his correct assumption that the ending was generally lost. But there were several positions where it looked like he could have an escape, and that's where it would have made sense to slow down and look. It's a risk, but there I think it's worth taking. Anyway, he's out, and Aronian advances.
The day's third winner was Maxim Rodshtein, who leveled his match with Vladimir Fedoseev. The game was an odd echo of the previous day's game: both won with Black after creating complications starting with a dubious ...g5 pawn sac. Fedoseev seemed too intent on playing for a draw - certainly in the opening - and it allowed Rodshtein to make lots of trouble for him. His reward: tiebreaks tomorrow.
The third player to advance is Vassily Ivanchuk, who was beating Anish Giri today, too, but he made Giri an offer he couldn't refuse: allow an immediate repetition or be dead lost. Giri chose to keep most of his rating points, and called it a tournament. Ivanchuk, meanwhile, will play Aronian in the quarter-finals in the only match that's set so far.
The other four games finished in draws and will result in tiebreaks. Alexander Grischuk vs. Maxime Vachier-Lagrave was a 13-move draw; apparently Grischuk is reverting to his gruesome but effective strategy from Kazan Candidates matches a few years ago, where he would draw all his classical games with White without a fight and then hang on desperately with Black, aiming to reach the rapid and blitz tiebreaks.
Bu Xiangzhi vs. Peter Svidler was also a short draw, but this doesn't seem to have been by design. Bu was outplayed in the opening, and was pulling on the emergency brake before things got out of hand.
Baadur Jobava outplayed Wesley So and had him on the ropes, but So saved the game by creating a fortress in the ending.
Finally, Evgeny Najer and Richard Rapport had a hard-fought draw. It looks like Najer generally had the better chances, but Rapport was never at death's door.
Games, with mostly brief comments, here.