The Four Sons of Horus were traditionally the guardians of the internal organs of the deceased. Each was associated with a particular organ, and also with a different cardinal point on the compass.
The Four Sons Of Horus
The Four Sons, from the British Museum
Coffin art detail showing the 4 canopic jars under the Lion Bier while the deceased is attended by Anubis and Horus.
See a full set of Canopic Jars on the NMS 4 Sons Of Horus Page.