Fourth Highest Leads
When leading a low card from a long suit it's normal to lead your fourth highest. Doing so will help partner know the length of your suit.
In this first example, North leads the ♥2 against West's 3NT contract. North has no cards lower than the 2 so he must have started with exactly four hearts.
North started with four hearts!
This time, North leads the ♥6 against West's 3NT contract. You can't see the ♥4 yet so you can't be sure about North's heart length. Once you see the ♥4 you'll know whether North started with four or five hearts.
Who has the ♥4?