Jamieson, Fausset and Brown on 2 Chronicles 9:25

25. Solomon had four thousand stalls--It has been conjectured [Gesenius, Hebrew Lexicon] that the original term may signify not only stall or stable, but a number of horses occupying the same number of stalls. Supposing that ten were put together in one part, this would make forty thousand. According to this theory of explanation, the historian in Kings refers to horses [see 1Ki 10:26]; while the historian in Chronicles speaks of the stalls in which they were kept. But more recent critics reject this mode of solving the difficulty, and, regarding the four thousand stalls as in keeping with the general magnificence of Solomon's establishments, are agreed in considering the text in Kings as corrupt, through the error of some copyist.