Also, what about Slashdot? The Jargon file? Usenet?

I've been on the Internet since the mid-90s and have purposefully avoided 4chan since I first heared of it.

At no point has it been difficult to avoid 4chan. I gather it has been very influential in the niche of image memes and I vaguely remember something about teenagers calling everybody a "fag" a few years back, but otherwise I couldn't say what the influence has been.