It seems like a lot of people instantly answer "4" and move on, but I don't know if the answer is all that simple. Let's consider what might actually be happening to everyone in the picture.
Number 1, the guy in the blue shirt who kind of looks like he's probably the kind of guy who smokes a pipe, is sitting on a branch doing nothing while numbers 2 and 3 are both trying to make him fall. We don't know how high the branch is, so the fall may cause minor injury or it may kill. Either way, number 1 is doing nothing to act against those who are trying to harm him. Is he a pacifist or is he just not paying attention to the world around him? Hard to say. If he's aware of what's happening but does nothing to stop it, then I'd say that's pretty stupid. If he's simply unaware of what's happening then he's also pretty stupid. Inaction, by choice or due to ignorance, is not a strong position to me.
Number 2 has the look of a guy who was once a high school bully but now is the old balding asshole who still takes pleasure in hurting others. We don't know why number 2 is sawing off the branch on number 1. The smile definitely makes it look like he's being a jerk, though it's also possible that, say, number 1 is a pedophile and number 2 is helping society make a hard but righteous decision. Again, it's hard to tell. However, since number 2 is looking at the branch he's sawing, we can say that he's aware that he's about to injure or kill number 1. Without having any justifiable reason for doing this, we can assume that 2 is a jerk. However, 2 also seems unaware that 3 is sawing the branch on which both he and number 1 are seated. Although the thickness of the branch may suggest that 2 will saw off number 1 long before 3 can saw off number 2, it still seems like 2 is oblivious to the fact that 3 is sawing the branch. His ignorance to his own situation while harming number 1 is pretty stupid, if you ask me.
Number 3 is the guy I'm most worried about here. We can't see his face, though he may be wearing a suit (maybe he's a Martin Shkreli executive asshole, kind of guy). Number 3 is sawing the branch of 1 and 2. Number 3 is not in danger himself (as far as we can tell). Number 3's action here will directly cause harm to 1 and 2 (well, as I said above, it may be more likely that 2 harms 1 first and then, after 1 falls, 3 will harm 2). Number 3 is looking at the branch he's sawing. He seems like he might be aware of the harm he's about to cause. What I see here is that 3 is about to injure or kill one or two people by choice. There could be good reason for it. What if 1 and 2 are both CEOs for health insurance companies that have been preying on the weaknesses of the nation while making themselves uber rich? Or, what if they're both serial rapists? However, without knowing more, it seems like 3 is the biggest jerk of them all. He's targeting others for harm.
Number 4, the one a lot of folks seem to think is the dumbest of the lot, is sawing his own branch and appears to be aware of it. He's looking down at the branch while sawing. However, he also has a smile. He could be unaware of the fact that his own action is about to cause him harm, but his smile makes me wonder if he knows what he's doing. Maybe he's committing suicide. Maybe he's harming himself on purpose. Maybe he knows the drop isn't very far and just wants to get away from the other idiots. Again, hard to say. If number 4 is unaware that he's sawing his own branch, then, yeah, he's pretty fracking stupid. However, if 4 is aware of what he's doing, then he may be the least stupid of them all. He's making a choice to do something (action), this something will not harm anyone else (1, 2, and 3 are not in danger through 4's actions), and he appears to be happy with the choice (not necessarily a good thing, but it could mean that he's found resolution in this choice). So, the only way 4 is the dumbest of them all, as most people say, is if he has no idea what he's actually doing. But we can't know that based solely on the picture.
So the choices become a little more difficult then. Which is dumber: inaction in one's own demise (number 1), causing harm to another while being unaware of your own danger (number 2), harming others outright (number 3), or harming yourself (number 4)? A lot of people think that 4 is the dumbest of them all, but I think that's only possibly the case if 4 is unaware of what he's doing. However, if 4 knows what he's doing, then I think 1, 2, and 3 are all far dumber than he. Number 1 is going to be harmed through inaction, number 2 is causing harm while seeming to be unaware of his own danger, and number 3 is about to harm others. But that's just my take on this situation (and making a lot of assumptions). What do you think?