When should a bridge defender return the second, rather than fourth highest card in suit?
The most common reason you'd lead back your second highest card from four would be to smother a relatively high card in dummy. With KJxx and 10x in dummy, leading the J seems right. This way you pick up the suit if declarer started with eg Q8x, and doesn't hurt on other holdings. But if dummy started with eg 6x (ie something that can't force your partner's high honor if declarer ducks) and your high spot is low enough that you won't block the suit when partner holds 5, it's important to convey the count to your partner by leading low.