I know it's comforting to believe that there were never any real racists on 4chan until relatively recently, and that it was all naive shitposters and kids making edgy memes, but it's far more likely that actual racists have always hidden behind the pretense of 4chan's ironic culture and anonymity, and have always been active there.

4chan still has a lot of good boards.

People have an impression that 4chan is exclusively /pol/ and /b/, but a lot of it's fantastic.

The folklore was that the neo-nazi forum Stormfront saw an opportunity and started astroturfing legit Nazi views on boards like /new/, causing this transition (and thus changing the course of western politics for years to come).

I never looked deeply into it, but I don't think the transition happened entirely naturally.