In game four Topalov has White, and of course he will have to try to get a good opening and get some winning chances. It will be very important for the players and their seconds to analyze the complicated game two, and draw the right conclusions.
I am not an expert on the Slav opening, nor do I want to become one tonight by analyzing all those complications, but my impression is as follows:
1.d4 d5 2.c4 c6 3.Nc3 Nf6 4.Nf3 dxc4 5.a4 Bf5 6.e3 e6 7.Bxc4 Bb4 8.O-O Nbd7 9.Qe2 Bg6 10.e4 O-O 11.Bd3 Bh5 12.e5 Nd5 13.Nxd5 cxd5 14.Qe3 Bg6 15.Ng5 Re8 16.f4
Previously, in this position 16..Rc8 and 16..Be7 were tried. Kramnik played the novelty
16..Bxd3 17.Qxd3 f5
Now the game continued
18.Be3 Nf8 19.Kh1 Rc8 20.g4 Qd7 21.Rg1 Be7
Now Topalov retreated the knight with 22.Nf3 and later found a fantastic Queen sacrifice, which however does not seem to win, but rather seems to be the best way in trying to safe the game. However here he could have played 22.Nxe6 and get good attacking chances after, e.g. 22..Qxe6 23.gxf5 Qf7 24.Rg4 followed by 25.Rag1.
Black could have avoided this on move 20 with a lot of complications after 20..h6 or 20..fxg4, but Svidler analyses the solid 19..Be7 and comes too the conclusion that "Black has very little to fear".
So it seems like Kramnik's novelty was actually a good one, and Topalov needs to play a different line tomorrow.
I think the main alternative against the Slav would be 6.Ne5, which Topalov has played several times with White. Kramnik has seen this line with Black a couple of times and played the sharp 6..e6 7.f3 Bb4 8.e4 Bxe4, which seems to be out of fashion at the moment in favour of 6..Nbd7. Certainly, Kramnik being in the lead would prefer that line with less complications.
Or maybe Topalov will play 1.e4, which I have discussed in the preview of game two.