>OP here. Why don't I fit into any one of those 4 boxes?
>I don't have an opinion on whether there is or isn't a god
Yes, you do. In not explicitly believing in a god, you are an atheist.
A claim to belief in god is exclusive. You either do, or you do not. "Not knowing" is an implicit disbelief, which is atheistic.
>I don't have an opinion on whether there is or isn't a god.
Then you do not believe there is one. "Not having an opinion" is implicitly an atheistic viewpoint, because it doesn't claim a belief. You HAVE to claim belief in order to be theistic; everything else, by default, is not theistic, which, you'll notice, is the literal definition of atheistic when broken down etymologically.
>a person who claims neither faith nor disbelief in God.
Key word being "claims neither", which, again, is atheistic, as it does not claim belief. It may not claim disbelief, but that's the catch most don't seem to notice.
Atheism, properly, is not belief vs disbelief.
It's belief vs. LACK of belief. There is no room for in-between there. An analogy would be "I have money" against "I have no money". You can complicate the argument all you want, with things that could be analogous to money, or money you owe/are owed, or assets that are worth money but are not money, or what constitutes money, or any number of other things, but, fundamentally, you can only give one, truthful answer.
Again, I fully understand what "agnostic" means in a colloquial sense, but the colloquial definition DOES describe an atheistic person, whether they know it or not. Similarly the colloquial definition of "atheist" is not complete; while most take it to refer to one who expresses explicit disbelief, you can be an agnostic atheist, carrying the implicit lack of belief (note, again, that this is not the same as disbelief) that comes with not knowing.