Part of me likes all the insanity with the internet back then, with mixed feelings. I probably got computer skills wasting time on YTMND for one reason or another. I 100% attribute 4chan's b for inspiring me to program. Their raids inspired me to learn programming when I was a teenager. But... I see that the alt-right came out of 4chan and the previously funny memes were no longer funny memes but serious accusations.…
I think you give too much agency to 4chan. It's just a imageboard - an internet forum - that happened to have some subforums related to alt-right. Maybe it had (has) a bad influence on people, but it's hundred times smaller than Facebook, Twitter or Reddit. To say it has single handedly started a movement is a huge stretch.
All you had to do was say "this will offend people" and 4chan rushed off to do exactly that, over and over.
4chan was originally just a contest to show how much something doesn't bother them, because they were still competing with each other like a clique of schoolkids, and any feelings were viewed as weakness.
Also, 4chan spawned QAnon. That was absolutely "starting a movement".