What are the 4th Official duties?

[dragonref]

I saw a pregame conference and there were four soccer officials. I have only seen three before - referee and two "line judges" (not sure of teminology).

So what does the fourth official do?

Also, I see where the line judges can talk to referee during game. Do they "suggest" calls that should be made that referee couldn't see. The referee has a very difficult job and can't possibly see all that is going on.