In Blackjack, you should never split 10s because they already form a strong hand totaling 20, which is tough to beat. A pair of 5s should also never be split, as it's better to double down for a total of 10 and try to hit a 10 or face card. Similarly, splitting 4s is not advisable, since you're better off hitting for a stronger hand rather than breaking them up. Lastly, avoid splitting 2s and 3s if the dealer has a 7 or higher, as their strong position makes it less likely that splitting will improve your hand.