Normally, a 2♥ open is preemptive. Weak. In 4th seat, this bid takes on a completely different meaning. It means you've got six hearts, and it's a stronger hand than opening. If you think about it, there's no reason to bid a weak 2♥ bid in 4th seat. . .you could just pass the hand out.

East, of course, has no interest in hearts with a singleton! But just in case West has spades, East will bid 2♠ to show at least five of them. Now West is happy to put East in a spade game and get a great score for the hand.

This is an example of a 4th seat opener at the 2-level. It shows a stronger-than-opening hand, with at least six in the suit bid. (There's no reason to preempt in the 4th seat, just pass it out and you'll likely get a good score.)

If East happens to open East's hand 1♦, South will overcall 1♥ to open the bidding for their side. Now, it's North who will jump to 4♥ and South plays the hand. But assuming everyone passes around to North, bidding what would normally be a weak preemptive bid (2♥) in the 4th seat is actually a stronger-than-opener. This bid allows South to go ahead and jump to game. . .note that South is already a passed hand - so if South invites with 3♥, that could be passed by North and you won't be in game.

In this case, South has a 7-loser hand (opening equivalent in terms of shape) and with 10 HCPs to boot South should go ahead and bid the game.

West isn't interested in a spade fit so much - especially with a solid heart suit (# of face cards from the top [4] plus # of cards in the suit [6] equals 10 or more). The question here is whether or not North will compete being non-vulnerable and bid 3♦. Those that do should be rewarded with a great score for N/S.

Although West is showing a stronger-than-normal hand by bidding 2♥ in 4th seat, East doesn't have a very good hand with which to compete - or even invite.