It says in the Book of Kings: 

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

Whereas in the Book of Chronicles, it says: 

“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.” (2. Chronicles 9:25) 

There seems to be a “contradiction” here as the first verse says 40,000 horses but Chronicles writes 4,000 horses. So how can we explain this discrepancy? 

The explanation is quite simple as it just requires careful reading. The first verse says: 

“… Solomon had forty thousand stalls of horses for his chariots…” 

The verse says that Solomon had 40,000 stalls for his horses for the chariots, so the number of horses is 40,000. 

The second verse says: 

“…Solomon had four thousand stalls for horses and chariots…” 

However, this verse says that Solomon had 4,000 stalls for chariots. So the number of chariots: 4,000 

Taken together, these verses reveal that Solomon had 40,000 horses and 4,000 chariots. That makes 10 horses per chariot. 

The chariots had five teams of two horses each. Why two horses? Having two horses pull a small chariot is stronger, easier and faster than just one horse pulling a chariot. Also, if a horse were to pull the chariot alone and died, the chariot would remain in the battlefield and the fighters would be trapped. But with two horses, the two fighters can still quickly go back with the other (second) horse and swap horses. (Similar to spare tires in a car race). 

And just as there were five fighter teams of two for a chariot, so were the horses. Because a horse (or both) could either become injured (1), killed (2), exhausted (3), tired (4) or in need of rest/food (5). It would therefore be problematic to leave the chariot dysfunctional. That’s why five teams have been formed per chariot so that they can switch among themselves at any time in one of the five scenarios mentioned above. In this way, it was guaranteed that the chariot always stays in battle to gain advantage over the enemy who is approaching on foot. 

Conclusion: 

The differences between the books of Samuel and Chronicles are not “Bible contradictions” but merely more information for the reader to get the full story and event.