Albert Barnes Commentary 2 Samuel 4:6

"And they came thither into the midst of the house, as though they would have fetched wheat; and they smote him in the body: and Rechab and Baanah his brother escaped." — 2 Samuel 4:6 (ASV)

As though they would have fetched wheat - This is a very obscure passage, and the double repetition in 2 Samuel 4:6–7 of the king’s murder and the assassins' escape is hard to explain. One interpretation is that Rechab and Baanah came into the house under the pretense of getting grain from the king’s storehouse, probably for the troops they commanded. In this way, they contrived to gain access to the king’s chamber.

Alternatively, they may have found the wheat-carriers—the people whose job it was to bring grain for the king’s household—just as they were entering the house. By joining them, Rechab and Baanah got into the middle of the house unnoticed. If this second interpretation is correct, a literal translation of the words would be: “And look, the carriers of wheat came into the middle of the house, and they (that is, Rechab and Baanah) struck him...”