Most people think 4chan is /b/, because that's where all the worst stuff happens. Imagine if reddit had an "anything legal goes" subreddit - it would be pretty messed up. Other boards can be pretty tame.
No, people think that because it's bigger than everything else put together, the fountainhead of the site's culture, and is the source of virtually everything notable that's ever happened there.
But I get what you're saying. /b/ really is the largest board. In a way it's meant to be the containment board, or the moat around the castle. It's where all the shit goes, so it doesn't pollute the rest of the site. And oh boy, is the internet full of shit.
4chan is divided into boards (like subreddits). For example, /pol/ is for politics.
/b/ is the "random" board (or "anything goes") and the biggest one on 4chan. Some pretty awful stuff happens on there (as well as some pretty cool stuff). Basically it's a grab-bag of Internet.
Reddit has subreddits dedicated to the foulest stuff that you'll ever occasionally see on 4chan. That has historically included borderline illegal content, with literally illegal content mixed in.
Reddit's reputation of being better than 4chan is entirely undeserved.
Both are interest-separated discussion boards. Reddit bills itself as more news-oriented, but quite often it serves as a regular threaded forum where the first post sets the context.
After reddit got big, we started seeing (well, started speculating that we're seeing) a lot of people come to 4chan after initially cutting their internet discussion teeth on reddit. There were speculations of a secret irc cabal of redditors, who collude to drive threads offtopic and in general mould board culture to be more reddit-like. To this day I'm not sure if those allegations were insubstantiated. We did have several prominent spammers who comandeered fleets of hundreds of proxies and spammed threads they didn't like with random-generated posts. These were obviously not redditors, but given there are people who care enough and have free time enough... In short, 4chan is a magical place.
