Who would Cain be worried about slaying him in Gen 4:14? Up till this point, all that are mentioned are Adam, Eve, Cain and Abel Were there other people outside of Eden before Adam and Eve had to leave?
It is inappropriate to read the Bible and assume that just because something wasn't mentioned, it must not have been.  The Bible asserts that Adam lived about 900 years.  A LOT happened during those 900 or so years, and yet we have a whopping two chapters in Genesis that deal specifically with Adam.  But, let's look at what facts we have.


  Gen 4:14 (KJV)&emsp;Behold, thou hast driven me out this day from the face of the earth; and from thy face shall I be hid; and I shall be a fugitive and a vagabond in the earth; and it shall come to pass, that every one that findeth me shall slay me.


The emphasized phrase doesn't make a lot of sense if "every one" referred only to Adam and Eve, who would appear (since no one else was listed) to be the only other people on the planet.  This is proven true a few verses later:


  Gen 4:17 (KJV)&emsp;And Cain knew his wife; and she conceived....


So, there was at least one other person in the land: Cain's wife, presumably a daughter of Adam and Eve.  However, even that's an assumption because no information is given about birth rates, gestation time, or anything else about humanity at a time when they lived much, much longer than we do today.  We don't even know how much time passed between the expulsion from the Garden and the conception of Cain.  It could have been minutes, but it also could have been decades with a great number of people being born during that time.

Indeed, the only fact we do know is that 130 years passed until the birth of Seth (Gen 5:3).  Presumably Cain and Able were adults when the murder happened (not necessarily, there are so many assumptions here!), and if so, then Cain was born sometime between year 1 and year 100 (starting from the Expulsion).  In either case, that's a lot of time to bear other children (and we have at least one: Cain's wife).

So, be wary about assuming that just because a useful piece of information wasn't presented (like a complete family tree for Adam), that it doesn't exist.  Cain had a legitimate fear, which suggests there were enough people that he couldn't easily disappear in his travels.