Technically it is both safe and non-safe in terms of images and such. 4chan is a collection of idiots/imageboards to discuss various topics.
Some of them are safe and posting 18+ content in them will get you banned. However some of them are actually 18+ only and that means you are allowed to post 18+ content in there.
It's also where kids go these days to pretend they smoke weed or had love.
I guessed it hasnt dawned on you kids our age have love and smoke weed?
4chan is fine, as long as you leave the memes etc on the site and not repeating them, it just is really annoying when people do that crap. Also, dont go on /b/ I made the mistake of going there when I first found about 4chan but I ended up being one of those people I mention use memes alot and was really annoying. I learned to shut the forget up with memes (unlike a lot of people in the blockland forum) and stopped going to /b/ but I regret being that way so much man