What is the difference between foursomes and four-ball?

Foursomes, also called alternate shot:

• The partners take turns to play first from the tee, hole by hole, and then alternate strokes until the hole is finished.
• Penalty strokes do not change whose turn it is.
• A stroke made in the wrong order is the general penalty. In stroke play it must also be corrected by the right partner playing from the same place.
• Either partner may act for the side before a stroke, such as marking, lifting or dropping the ball.

Four-ball:

• The side's score for a hole is the lower score of the two partners. If only one holes out, that score counts.
• One partner may play alone for the side. A partner who arrives late may join only between holes.
• The partners may play in whatever order the side thinks best.
• A penalty normally falls only on the player who broke the Rule, not on the partner.

In both:

• Partners may give each other advice.
• Partners may share clubs, as long as they have no more than 14 between them.
• A player may not stand on the line of play behind a partner who is making a stroke.

The two formats test different things. Foursomes makes each player live with the partner's shots. Four-ball lets one partner attack while the other plays safe.