Jacob picked Kate to be #4 this time through... by Ben-Locke

This theory has to do with why we didn't see a number listed for Kate in the cave that Fake Locke took Sawyer to in "The Substitute."

I feel strongly that Jacob and MIB are stuck in some sort of time loop that Jacob created to stop the end of the world. For whatever reason, I can't imagine the end-game of the show being anythings less important than this war they all will fight saving humanity from certain destruction. Seems to me that Jacob and MIB are from the future and KNOW what is going to happen. Jacob believes that he can find the right combination of people to stop the destruction to come. MIB believes that, no matter what, the people that he brings to the island will eventually all corrupt. He wants to leave the island and live his life as fully as he can, despite knowing that the end is coming. But Jacob keeps making them go back to the beginning and keeps working to get it right. I believe he keeps realizing, somehow, that he needs different people to be there to get just the right combination of forces to be able to stop the end that is coming. Somehow, he is able to assign each person a digit ! corresponding to the Valenzetti equation.

In the show that we are watching, we are seeing the most recent iteration of Jacob and MIB going through the time loop. We have just seen MIB cross off Locke's name. Obviously, Locke was not the right person for Jacob to have in the equation in order to stop the end. I believe that, this time, Jacob has realized that he needs Kate, and that we will discover that he has brought her to the island for the first time in this iteration through the time loop.

Sawyer seems to be joining Fake Locke on the promise of getting off this island (I know some don't believe him, but I believe Sawyer was sincere in telling Fake Locke "Hell Yes"). We know that Jacob is expecting one of his chosen to step up and take his role as the principal protector of the island. I believe that Jack will be that person. He will act on faith and decide he can't let anyone hurt the island. We saw the ghost kid tell Fake Locke that "you can't kill him." I believe that ghost child was Jacob, appearing as a child as he's been freed from the time loop by death, and I believe he was telling Fake Locke that he can't kill JACK. However, Fake Locke is going to manipulate SAWYER into killing Jack. I believe that Jacob, from living through the time loop the previous time, knew that this would happen. He has brought Kate to the island so that she can stop this from happening. Quite literally, she will stop Fake Locke from CORRUPTING Sawyer and this will, in turn, save! Jack from death and allow him to stop the end that is coming. This will be Kate's major purpose in being on the island (and I believe we will see that each of them has some sort of purpose in stopping the end from coming).

At some point, I believe we will see Jacob, or some form of him, write Kate's name on the cave and put the number 4 next to it.

Thoughts? Am I crazy? This is my first theory, and, like everyone says, sorry if it's close to anything that's been posted previously.