Albert Barnes Commentary 1 Kings 4:26

"And Solomon had forty thousand stalls of horses for his chariots, and twelve thousand horsemen." — 1 Kings 4:26 (ASV)

In 2 Chronicles 9:25, the number of stalls for Solomon’s chariot horses is stated as 4,000, not 40,000. The number in the present passage is likely a scribal error. Solomon’s chariots numbered only 1,400 (1 Kings 10:26; 2 Chronicles 1:14), for which 40,000 horses could not possibly have been required.

Assyrian chariots had at most three horses each, while some had only two. A force of 4,000 horses would supply a full team of three for 1,200 chariots and a smaller team of two for the remaining 200. The number 4,000 is in proper proportion to the 12,000 horses for the cavalry and is consistent with what we know of military establishments of that time and region (compare 2 Chronicles 12:3; 2 Samuel 8:4).