Albert Barnes Commentary 2 Samuel 4:2

"And [Ish-bosheth], Saul`s son, [had] two men that were captains of bands: the name of the one was Baanah, and the name of the other Rechab, the sons of Rimmon the Beerothite, of the children of Benjamin (for Beeroth also is reckoned to Benjamin:" — 2 Samuel 4:2 (ASV)

Beeroth - See the cross-reference. From Joshua 9:17, one might expect the population of Beeroth to have been Canaanite. However, for some unknown reason, the Canaanite inhabitants of Beeroth had fled to Gittaim—perhaps the same city as Gath—and remained there as sojourners.

If this flight took place during Saul’s cruel attack upon the Gibeonites (2 Samuel 21:1–2), then Baanah and Reehab may have been native Beerothites. This would suggest they were motivated to murder Saul’s son by a desire to avenge the blood of their countrymen.

The fact that they were considered Benjamites is entirely compatible with them being Canaanites by blood.