iPhone 4S Safari Scrolling
If you were on a Mac I would say that you had turned off the Inertial Scrolling preference, but there is no such setting for iOS devices running Safari, and it should be the default behaviour at all times.

The answer by @Aaron is a good start to force quit an App (note this works for all apps, although in many cases the app isn't really running in the background, it just has a saved state that allows to a fast restart that you can clear using this method).

If it's still doing this after you have restarted it, check for other scrolling characteristics to see if there are other issues.  If you scroll past the top/bottom of a page, and let go, does it "bounce back" (rubber banding), or can you not extend past the edges?

Other than that, you can work on your flick technique, as it were, release your finger before it gets to the edge of the screen, can you get different distances each time, or is it a pretty much static scroll?  You should be able to get anything from a few lines to a few pages.

Lastly, and this isn't in your question but it related to scrolling large distances, when at the bottom, you can scroll to the top in a single stroke just by tapping the very top of the screen; this should work in any vertically scrolling app (Twitter clients, etc), not just Safari.