Is 4SF “asking” or “telling”?  
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.