The fouth divergent universe is interesting because Stevens begins using his knowledge from previous trips to anticipate and manipulate events around him. He avoids the spilled beverage, has his ticket ready for the conductor, and jumps ahead of the conversation. He is stopped while trying to get the gun, and that's the end of that world for him when he again dies in the explosion.
It is in the fifth world that information starts flowing to him in ways that change his circumstances. He learns that the patch on Goodwin's shoulder puts her at Nellis Air Force Base, and so knows where to find Dr. Rutledge. He also learns, through Christina's smartphone, that he, Captain Colter Stevens, was reported dead in combat in Afghanistan, that he saved the life of one of his men (Sargeant Robert Alvarez), and that his father Donald received the Silver Star awarded to him posthumously. It is this latter information that leads to his confrontation of Goodwin, demanding to know whether he is or is not alive, and the consequential weakening of his imagined reality discussed last time.
This time, though, he places a phone call to Nellis Air Force Base and leaves a message, including his real name, for Dr. Rutledge. This is the first time he has done something as Fentress that might alter events for the Stevens in the newly created universe. Up to this point, he either died (along with nearly all witnesses) on the train or was killed immediately thereafter, with his actions easily attributed to his distress over the realization that there was a bomb on the train. The call to Dr. Rutledge, though, is entirely different. We have to assume that Rutledge gets the message, that the man who is at that moment lying barely alive with half his body missing on a life support table in a room in his offices placed a phone call to him that morning.
Of course, Rutledge could dismiss it as a prank: someone who knew the identity of their top secret subject called pretending to be him. This Rutledge does not know that he lives in a divergent universe created when a divergent self sent a divergent Stevens into the body of Sean Fentress on a train moments before it exploded, and he believes that time travel is not possible and that even if he does send Stevens to "the past", Stevens can't call him. However, moments after the call is logged (and the times of these events will be logged accurately) a train bombing occurs, and the bomber warns of a coming second bomb in a way which launches Beleagured Castle into its first mission--sending Stevens back to that time. It could begin to fray the edges of his conviction that the world to which he sends his investigator is completely unreal.
That creates unpredictable results, because it means that in that universe Rutledge is changing, and that means his interactions with everyone, and particularly with Stevens and Goodwin, are also changing, and that their situations are correspondingly altered. To this point we knew that every universe Stevens created was similar enough to the original that we could predict identical universes being created from each of them. Now suddenly it is possible that Rutledge will do something different which will mean Stevens will do something different, which means that there might be a world we did not predict.
That problem escalates if it starts small and does not have any impact immediately. That is, Rutledge was told that Stevens called, then he realized that he was sending Stevens to the past, but he considered it a coincidence. If it does not change anything this Stevens does as Fentress until his fifth trip, then when he returns and informs Rutledge that he made that call, Rutledge will already have received it that morning and will be forced to confront the change. At that point he either arrogantly assumes that he is right, or he changes his strategy; and if he changes his strategy, we may have worlds we did not wish to have.
That we will consider next time.