2 Chronicles 9:25

25 And Solomon had four thousand stalls for horses and chariots, and twelve thousand horsemen; whom he bestowed in the chariot cities, and with the king at Jerusalem.

1 Kings 4:26

26 And Solomon had forty thousand stalls of horses for his chariots, and twelve thousand horsemen.

The account in 2 Chronicles says 4,000 stalls of horses, the account in 1 Kings says 40,000 stalls for horses and chariots.

One solution is to propose that it was 4,000 structures, each with 10 stalls for horses, and the accounts are using "stalls" to refer to two distinct things - one the structures, the other the stalls within the structures. In other words, the larger structure could be called a "stall", and each slot within them can also be called a "stall".

Another solution is to state that the account's description of "4,000 stalls for horses and chariots" (2 Chronicles 9:25) is designating one structure per chariot team (10 horses/team, 10 stalls/structure). The account in 1 Kings does not say "for horses and chariots", but rather "of horses for his chariots".

Lastly, we can recognize that if Solomon had 40,000 of something, it's also true that he had 4,000 of that same thing, and one account could simply be being less precise.