4PC on Chess.com uses the Glicko2 rating system, the same as regular chess. However, the rating changes are calculated a bit differently, since you have 3 (or 2) opponents instead of 1!

In Free For All mode, each player has their rating adjusted based on the average rating of their 3 opponents.

For example, if there is a 1500, 1600, 1700 and 1800 playing a game, then for the 1500, their rating change would be calculated as if they were playing against a 1700 (the average of 1600, 1700 and 1800).

But for the 1800, the rating change would be calculated as if they were playing against a 1600 (the average of 1500, 1600 and 1700).

The rating adjustment also varies based on what place you take.

1st place gets their rating adjusted as if they won 3 times against the calculated rating.

2nd place gets their rating adjusted as if they won 2 games, and lost one game against the calculated rating.

3rd place gets their rating adjusted as if they won 1 game, and lost 2 games against the calculated rating.

And 4th place gets their rating adjusted as if they lost 3 games.

Teams rating adjustment is done just like a 1v1 game, except the rating of the teams is calculated slightly differently.

For the team rating, it is not an average of the two team members' ratings, but is instead weighted toward the higher rated player. The calculation is done like this, with the higher rating being H and the lower rating being L:

(2H+1L)/3

so if team A, an 1800 and a 1300 were playing versus team B, a 1500 and a 1600 then the rating adjustment would be calculated as if a game between a 1633 player (Team A) and a 1566 player (Team B)