It is best to play blackjack according to basic strategy. While using basic strategy is the best way to impact the house edge and make the best statistical play possible each round, it does not often allow for a lot of maneuvering or for any little tricks.
But there are times that a blackjack player can deviate from basic strategy a little bit to take advantage of a house rule. One such rule is being allowed to double down after splitting pairs, but there is only one real hand that can take advantage of this rule, and that hand is a pair of 4s.
Normally basic strategy advises to hit on a pair of 4s rather than split them. This is because there is not advantage to splitting them.
However if you are allowed to double down after splitting there are two instances with a pair of 4s that you should split, and that is when you are faced with a dealer’s 5 or 6.
Once you split those 4s you will receive another card so that each hand will have two cards again. If you are dealt a 5, 6 or 7 you will have a hand total of 9, 10 or 11. If the dealer is showing a 5 or 6 it stands that there is a pretty good chance the deck is running with lower cards, which will help when splitting those 4s.
If, after splitting and receiving a second card for each hand, you have a new hand total of 9, 10 or 11 it is time to double down. Again, if you look at a basic strategy chart you will see that the best play for a hard 9, 10 or 11 is to double down. This is because doubling down on a 9, 10 or 11 is offensive in blackjack and you stand a good shot at making twice the winnings.
So to review, when the house allows you to double down after splitting pairs and you are dealt a pair of 4s, it the best play to split those 4s. If after splitting those 4s either one of your new hands has a total of 9, 10 or 11 you need to double down.