Fourth Suit Forcing is a convention that every club player will have heard of but will not necessarily make use of as often as they might. Some players seem to actively avoid it, perhaps feeling that their partnership knowledge of the convention is not what it should be.
FSF is often introduced as an asking bid - the FSF bidder will often be looking to play in a no-trump contract but only if he can establish whether the partnership has a stopper in the fourth suit. His FSF bid asks partner whether he has a stop in that suit. If so, partner bids no-trumps next, jumping to Three No Trumps, if that option is available, to show a stopper in the fourth suit and a strong hand. With no stopper in the fourth suit, partner will rebid one of his suits or else support my first suit with (usually) three-card support.
Here are some other bits of info that you might find useful.
1) A FSF bid is not just an asking bid, it is also a 'telling' bid. Specifically, it tells partner that you do not have a suitable alternative natural bid available.
Example
QJ63 A1095 72 KJ8
Partner opens One Diamond, you respond One Heart and partner rebids Two Clubs. What do you bid next ?
It is wrong to bid Two Spades. You have a perfectly descriptive bid of Two No-Trumps available, showing 11-12 points with a spade stop. Perhaps you want to bid Two Spades to see if partner also has four of them ... maybe even spades is your best trump suit ? Forget it, he doesn't have four spades. He'd have rebid One Spade, not Two Clubs, if he did.
Contrast that example with this one :
J93 A10952 A8 AJ4
The auction begins as before. The difference here is that you do not have a suitable natural bid available at your second turn. The shape of your hand, and high-card values, suggest a game in no-trumps might be available but only if your side has a stopper in spades. Or maybe partner has some heart support for you and a game in hearts will be a better contract ? Or maybe partner has ten or eleven cards in the minors and a minor suit game (or slam) is where you belong ?
You need more information about partner's hand and this is the time for a fourth suit bid of Two Spades. If partner can bid no-trumps next you will play in Three No Trumps. You will raise a Three Hearts bid to Four Hearts. If partner rebids a minor suit, you will look at reaching a minor suit game.
2) After One Club - One Diamond - One Heart, responder's rebid of One Spade is a natural bid showing four spades. It seems wrong to play this as a fourth suit bid. There could still be a 4-4 spade fit so it seems best to be able to bid a natural suit at this stage. If you do decide that a fourth suit bid is needed then you bid Two Spades :
A5 Q84 KQ1072 K53
You want to reach game but in which denomination ? You are not sure so you bid Two Spades, asking partner to tell you more.
You might argue that you have the values, and the spade stop, for a natural bid of Three No Trumps. True but partner is very likely to Pass this and you are not sure to be in the best game contract. After the likely spade lead you will probably need to run nine fast tricks and that might not be possible. Most of the time, if Three No Trumps is going to make, you will need partner to lend a hand in spades.
3) How forcing is forcing ? Most FSF bids are made at the 2-level and "forcing to 2NT" is a popular arrangement. However, it is more useful to play FSF as "always game-forcing". Now you don't have to worry about partner Passing a bid before you reach a game contract.
J53 J8 KQ1072 A105
After One Club - One Diamond - One Heart, you are not strong enough for a fourth suit Two Spades if you play this as game forcing. Instead, you'll have to rebid Three Clubs (10-12 points with club support). This is not a forcing bid.
A small change to your hand gives you a tougher problem :
J53 J85 KQ1072 A10
For me, the least bad choice here is One No Trump.
4) Try not to tell partner what he already knows.
K4 AJ983 92 KQJ6
After One Heart - One Spade - Two Clubs, partner continues with a fourth suit Two Diamonds. What next ?
Partner already knows you have at least five hearts and four clubs. Partner does not know you hold Kx in spades so bid Two Spades now. You might well have supported partner's spades on the previous round if you'd held 3-card support so bidding Two Spades now suggests a 2524 shape.