for all you newfags not getting this:

4chan is /b/.
You do not split /b/ from 4chan, you split the other boards from 4chan, because you cannot split 4chan from 4chan, which in case you didn't get; IS /b/.

In the beginning there was /random/ ,then /a/ where the community grew from (mostly FYAD in /b/-random and ADTRW posters in early /a/)and also MOTD's on Raspberry Heaven and a few GBS's helped to spread the word and combined made 4chan what it sadly isn't today.
(most of the users today dont even know who Hiroyuki Nishimura is.)

The other boards came later.. some of them had to split due to obvious reasons, /g/ and /l/, but never /b/.. /b/ was like a mixture of /g/ /h/ /l/ but it never split, why?

Because /b/ is 4CAHN.