1. The Vaticanus has conveniently left this verse out in 1 Kings 4:26, and the Hebrew has 40,000 stalls for horses.
In 2 Chronicles 9:25 the Hebrew has 4,000 stalls and the Vaticanus has 4,000 mares for chariots. So the Vaticanus had avoided, what they perceived to be a problem, altogether.
Since any Greek is merely a translation from the Hebrew, one would naturally assume the Hebrew to be correct over the Greek. So how do we begin to solve the problem of 40,000 stalls versus 4,000 stalls. We start with the word stall. The Hebrew word is "urvah" and is found only three times in the bible. One of the times is found in 1 Kings 4:26, the second in 2 Chronicles 9:25, and the third time in 2 Chronicles 32:28. According to 2 Chronicles 32:28 this could be an individual stall or a complete stable. So you would have to determine if a single stall, or a stable is being referred to.
1 Kings 4:26 reads different than 2 Chronicles 9:25. 1 Kings 4:26 reads 40,000 stalls of horses for his chariots and 2 Chronicles 9:25 says 4,000 stalls for horses and chariots. The 40,000 could be individual stalls for 40,000 horses, while the 4,000 stalls could be 4,000 stables containing horses and chariots. Now I agree that the difference between 40,000 and 4,000 could be merely the absence of a little old zero, and the assumption would most likely be that. Yet the potential is there for it to be 40,000 individual stalls and 4,000 stables.
If you have 12,000 horsemen plus 1,400 chariots you would need 40,000 individual stalls. How many horses did each chariot have, two, or four, or six? The 12,000 horsemen are not likely to be the chariot drivers as observed by other scripture. So 12,000 horsemen need how many horses? And 1,400 chariots have have how many horses, 2,400, 5,600, or 8,400? Consider that you have breeding mares, and breeding stallions, and then you have foals. 40,000 seems like a realistic number to me, having had as many as 45 horses at a time.
The Jews fought against 1,000,300 Ethiopian chariots at one time. Solomon had 80,000 men cutting wood for stables and other projects. Forty thousand stalls does not sound like a lot when you consider that.
2. So what do we have? The NIV put 4,000 in place of 40,000 in 1 Kings 4:26, while neither the Hebrew text nor the Vaticanus allows for it. So why did the NIV put 4,000? Did they just decide in their own minds that they would change the Bible, contrary to what all other translations had written? This is called adding and taking away from the Word of God.