40,000 (1 Kings 4:26) - "And Solomon had 40,000 stalls of horses for his chariots, and 12,000 horsemen."
4,000 (2 Chron. 9:25) - "Now Solomon had 4,000 stalls for horses and chariots and 12,000 horsemen, and he stationed them in the chariot cities and with the king in Jerusalem."
There are two possible explanations for this discrepancy. 1) a copyist error. 2) the difference is due to time; that is, one account is at the beginning of Solomon's reign (1 Kings 4:26), and the other at the end (2 Chron. 9:25). I believe the most probable is a copyist error since we can see that Chronicles does have copyist errors in other areas. Therefore, it is probable that the same thing occurred here.

"In general it can be said that the books of Chronicles furnish approximate numerical estimates in the form of round numbers, frequently designed, as has been remarked, to express the magnitude of the occasion....Some estimates in Chronicles which appear to be particularly inflated can be corrected or scaled down by reference to the books of Samuel and Kings...However, it is not always the case that the figures in Chronicles exceed their counterparts in Samuel and Kings."1 The correct answer is probably 4,000 since 40,000 seems extraordinarily large. Furthermore, it seems likely that a single "10's" place was copied incorrectly accounting for the discrepancy.

Usually does not cause any crises of faith with anyone except people like 嘁哩喀喳 who likes to rinse and repeat the same things hoping to win an argument with no end :)