The team with -4 is the favorite. It means they have to win by more than 4 points. The team with +4 is the underdog and is getting points. They can lose the game by less than 4 points and people betting on them will win.

The team that has the +4 is the underdog, and you will add 4 points to their score.
The team with the -4 is the favorite to win and if you bet them you will need to deduct 4 points from their score.
For example, if you pick the underdog team, you get to add 4 points to their score.
So if your team loses 20 to 17, you win, since you get to add the 4 points to their total, bringing it to 21.