In this sequence:
1♥ 1♠
2♣ 2♦
the 2♦ bid is a new suit by Responder and Opener must absolutely bid again.
In standard bidding that 2♦ bid would be showing a ♦ suit, probably in hopes that Opener would be able to bid Notrump.
But when you think about it, if Responder actually held ♦s he could have just bid Notrump himself, so in this sense the bid is not very useful.
Using Fourth Suit Forcing Convention the bid of the fourth suit takes on a new, artificial, meaning. It now says: "Partner, I am strong enough to make a bid but don't have anything I can say, so I'm just bidding the fourth suit. I want you to describe your hand further."
Assume that with the bidding sequence above you held this hand:
♠ A Q 9 8 4 ♥ K 3 ♦10 7 3 ♣ A 10 4
You cannot rebid your ♠ suit without 6 of them, you cannot support ♥s with only 2, you cannot support ♣s with only 3 and you cannot bid Notrump without a ♦ stopper. So what are you going to bid with your 14 points? Of course, you bid 2♦, Fourth Suit Forcing.
Opener has certain priorities to follow after partner has bid the fourth suit.
- support Responder's first suit with 3 of the suit
- bid Notrump if she has a stopper in the fourth suit
- find some other bid if she cannot do either of the above