Albert Barnes Commentary 2 Samuel 21:22

"These four were born to the giant in Gath; and they fell by the hand of David, and by the hand of his servants." — 2 Samuel 21:22 (ASV)

Four. This does not necessarily mean that they were brothers, but that they were all of the race of the Giant, all Rephaim. The word “four” is omitted in the parallel passage, where only the last three are mentioned.