I choose option 4. I have knife, a pistol and 8 ammo, and water purification tablets, and I need to eat prior to the end of day 4.

The statement that not everything needs to believe the most likely story sounds kind of like an ominous warning. The "most likely" story is that we will have "fellow player", with a chance to team up. In rejecting the claim that it is a "fellow player", and we then assume that he is dangerous, he will try shoot me no matter what-so basically do not approach. If we consider my thinking is backwards, and that he really is a good guy who would travel with me and pool resources...well the only thing i really need is food, err weeds so I don't really need him and I don't really need him alive either. Maybe he has water, but I have water purification tablets and I probably could collect morning dew [in the morning] once I am in the woods.
Considering I need food by the end of next round, especially if I can't find edible greens, teaming up could still be an option. So in teaming up, if he doesn't have food, I would have to kill him immediately [as he is food] when he is off guard to ensure he doesn't have the same ambitions I did. The option for now is just teaming up, so he gets the opportunity to strike first. Even if he pretends to be buddy buddy, even if he is unarmed, he could, while I am sleeping, take my pistol and kill me and its game over man, game over.
The best options are to shoot him or run. I don't like killing people, and even in fighting there is so many things that could go wrong. I mean I had to steal this gun as I didn't have my own, and never done any target practice with it and probably with any gun-but ?per day 2?-we can assume it and the bullets are functional. If he has a gun, it could be his, he probably knows how to use it, and may know how to fire it quickly, and maybe has some muscle memory so that he doesn't have to aim it to hit a target. trying to shoot him I think is a bad idea. I stand behind the non-aggression principle, and just run away. In running away, if he sees me it shows him that I am not hostile at least. Still he could shoot me in the back, but the further away I am then hopefully the less accurate he is.