The purpose of 4SF is to create a game-force at a low level so that you and your partner can figure out:
1) where to play your game; and/or
2) whether you have slam.

If you know where the contract belongs, just bid it –don’t use a convention just because you play it!

1) 4SF is a bid of the 4th suit at the 2-level or 3-level.  1♣-1♦-1♥-1♠ is NOT 4SF.  1♦-1♠-2♣-4♥ is not 4SF.
2) You only use 4th suit forcing when you have enough points for game AND:
    a. You are unsure what strain to play the game in; and/or
    b. You are interested in slam.

People always ask me whether 4SF is “asking” for a stopper or “telling” about a stopper.  I always tell them “neither”.  They look at me like I am crazy; but it is the truth.  4SF is simply an artificial game-forcing bid that tells partner that we have enough points for games but that we need more information to figure out where that game is.  You might bid 4SF with the fourth suit stopped because you are looking to see if partner has 3-card support for
your major.  On another hand you might bid 4SF with only 4 in your major to find out if partner has a stopper in the fourth suit.  On another hand you might bid 4SF with only 4 in the major and a stopper in 4th suit just to game force cheaply so you can show support for opener’s suit and explore for slam.  So the answer to this common question remains “neither” and you should view 4SF as a convenient way to game force that says nothing about stoppers.

In response to 4SF, opener should make a bid that further describes her hand.  Typically she shows them in this order of importance:
1) 3-card support for partner’s major by bidding partner’s major;
2) A stopper in the 4th suit by bidding no-trump;
3) Support for partner’s minor;
4) More length in her own suit by bidding that suit again;
5) A natural third-suit by bidding it.