If still tied 2-2 after four total rapid games, then a pair of blitz games at 5+3 will occur. If no winner, four more pairs of blitz games. The two would have to tie after five pairs of blitz games (10 total blitz games) before an Armageddon game would be played.

https://www.chess.com/article/view/world-chess-championship-2018-carlsen-caruana