Delay the offence (to give time for additional defenders to arrive);
Deny high percentage shots and force the offence to take lower percentage shots;
Pressure the player with the ball into making a poor decision.

4v3 is best illustrated in the context of transition.

(English) When defenders are outnumbered 4v3, they must:

Put pressure on the ball;
Deny any lay-up or shot from near the basket.

(English) Most teams will align with 1 player on the ball and the other two players on the high or low split line.

(English) The defender on the low split line (x3) must deny any post player, whether they have run to the “rim” or are in the low post.

x1 can also hedge towards the basket to assist in defending that area.

Where 4 has run to the “rim”, x1 can remain high.

(English) However, if 4 is in the low post, x3 must front and x1 must “sink” to stop any lob pass.

(English) When the ball is passed to 1, x1 should not close out to the ball. If they do, 4 may “seal” x3 and receive an easy pass for a lay-up (as x3 is on the “outside” of 4).

(English) Instead, x3 closes out to 1, x1 rotates across to stop any pass to the low post and x2 drops to the high split position. All defenders must move with urgency.

(English) Where an offensive team does not send a player to the basket but instead has a strength in perimeter shooting, the defenders may align differently.

The ball must still be pressured, however x2 and x3 may align horizontally which will enable them to cover perimeter shots more easily.

(English) If a team adopts this type of defence the offensive team may:

Drop the perimeter players into the corner in an attempt to separate from the defender. The defenders may drop down the key, but should both sink to the basket;
Drive from the perimeter. If they do this the ball side defender (x2) does not have a good position to stop the drive to the basket. Instead x3 must sprint across to stop the penetration.