This showed up in my news feed today, which caused a bit of an argument between those who considered themselves a 3 and those who considered themselves a 4. I decided to clear this up.
The people who consider themselves #3 consider their position to be the most intellectually honest. Often, they will become armchair philosophers, citing that we cannot really know anything. According to this philosophy, I cannot know, for example, that my car will start tomorrow morning. I believe it will start because it started this morning and yesterday morning, but all I know from that is that it had started, not that it will start. An agnostic believes that the existence of a god is not knowable, because it's possible that god is outside our human understanding.
This seems logical enough to the armchair philosopher, but it misses one critical element: If the existence of a god could be outside our understanding, how is it that certain people have made the claim of the existence of a god? If it's outside of human understanding, then these people should have no knowledge of the existence of a god. However, this is not the case; entire religions have been built around this knowledge of the unknowable. How can someone have knowledge of something unknowable, when everything we know is clearly within the realm of our understanding?
Consider a favorite magic trick of mine. Suppose I make a red handkerchief disappear in the palm of my hand, right before your eyes. You have seen something that had clearly existed vanish into thin air, contrary to the law of conservation of matter. When you ask me how I did that, I tell you, "It's magic." In other words, it's something beyond your understanding and supernatural. Except it isn't beyond anyone's understanding, and is clearly not supernatural, I just wanted to fool you into thinking I had magical, mystical powers that make me special. I know that there was no magic, but, at the very least, you must remain agnostic about the possibility of my magical powers if you cannot figure out how I performed this simple trick.
Of course, the more rational among you will realize that all magic tricks are really just illusions performed by some sleight-of-hand, mirrors, or some other trickery, and cannot be agnostic on this topic. You realize that, even though you don't understand how the trick was done, it was just a trick. You don't remain agnostic about magic if you are truly intellectually honest, because you can go to the library, read about magic and how it's performed and learn how these tricks are done.
A person who knows there are no gods has arrived at this conclusion the same way a person might arrive at the conclusion that there's no such thing as magic, while the intellectually lazy, who might consider that a stage magician may actually be performing real magic, suggests that it's possible that a god might exist outside of our knowledge. Those of us who investigated and know how and why the god trick is done cannot be agnostic about it; we know that no gods exist the same way we know there is no magic, because, in spite of what the lazy armchair philosopher might think, things really are knowable.