game 4 key moments

Position 1 after 17Nd3;Black to play

Karjakin

Carlsen

Carlsen just played 17Nd3

Solid position for white, controlling center and d5 square.

Karjakin tried to attack with..h5 

Position 2 after 34Nb5, Black to play

Karjakin

Carlsen

Here Black should play 34..Ra1 and its not clear what White should do. 35 Ra1 or 35 Kh1 waiting;
Instead 34..Rb3?! ran into 35 Nd4 forking Rook and Queen. White gains the exchange while still being in control

Black to play after 48 Kh1

Karjakin

Carlsen

Here Black played  48..Qf2?? which ran into 49Rc8+ Kh7 50 Qh6+!! Kh6 51 Rh8 Checkmate or gh6 51 Rf7 Checkmate

Black had to exchange queens but ending is riskless for white. The critical mistake was to sacrifice the exchange on move 34; 34..Ra1 was the only way to keep the game going for Black.

The final combination is easy to spot, and it is possible Karjakin saw it but went for anyway.