EDIT: I spend a metric forever editing it for readability, so it's kind of out of place with the most recent replies. But it does answer many points raised there, it just doesn't fit textually
3 and 4 safe, 1 is borderline, rest questionable. I'd like to point out this in particular means the last one is not explicit. Just because there's a hint of pubic hair visible it doesn't make things explicit. Even full frontal nudity isn't explicit by itself, as long as it's not ostentatious (or particularly loli, which IMHO lowers the bar for what counts as ostentatious considerably).
When rating, keep in mind that child-friendliness or work-safety are not the purpose of the system, for the simple reason that danbooru never will be truly safe for kids[1] or work, and trying to make it so is a stupid waste of time and resources. Instead it's a means to classify content into three distinct groups depending on what the people browsing are interested in:
1. Completely clean stuff with no sexual overtones whatsoever (for safebooru users)
2. Somewhat or even heavily erotic stuff for people who don't mind sex, but without porn polluting the list
3. The rest
All of the above are targeted at sane adults[2] who understand that sometimes, when two or more people love each other, they will fuck each other senseless, and aren't offended by that. There's also the unfortunate yet unavoidable problem that 3. lumps together plain porn with, shall we say, "explicit erotica", for lack of a better name. I'd gladly banish the former while leaving the latter, but there's no technical or even intuitive criterion to distinguish the two. For instance, I personally consider all of kaieda_hiroshi's explicit works awesome and amazingly tasteful, even though they're full of bodily fluids, and there's just no way to define it. So as much as I'd rather not, we have to live with porn being here, and the only solution is to exercise unrelentingly strict quality control.
So, uh, that turned out to be rather long, but I've wanted to bring the topic up for some time now. It's been bugging me that so many questionable posts are misrated as explicit.
[1] See [2] for my opinion about the whole idea of doing that all.
[2] I'm also okay with unusually mature kids and even horny 13-yo full of hormones, as long as the latter STFU and keep to fapping. I'm heavily opposed to idiotic legal limits as a crutch for the inability or unwillingness of parents to raise their kids and explain sex to them properly.