I think these four for very different reasons each.
"while Jack is annoying as all heck with his attitude, I never thought of him as bad. Though he is Locke's nemesis or the Yang to Locke's Yin, in a way.
--Sawyer can easily fit the "bad" mold (but I love him anyway).
--Kate has been very bad in her past, but it wasn't like she murdered an innocent person.
--Hurley seems all around nice guy, no threat -- so why is he among the four??"
Jack- He is a Dr. and seems to be a pretty good one. I can see why they would want him with his medical experience.
Sawyer- He may be "bad" but he is smart. He horded the guns, the medicine, some food, and who knows what else. He used them as bargaining because he did not have anything else. He is a con man who plays his game well.
Kate- Tough person who will do whatever it takes to reach her end goal. She is smart, and resilient, and strong.
Hurley- I want to think that maybe it has something to do with his mental illness. Some people believe that those with the type of illness he has can see an alternate dimension, somewhat like small kids can see their guardian angels if they are never told they do not exist.
I think they each offer something that is needed for the continuance of the others existence.