4chan surprisingly is not a free speech platform, nor did it ever pretend to be.
It has fairly bog-standard rules about not being a hateful jerk, it's just that enforcement of those rules is pretty spotty so the sites users consist mostly of hateful jerks.
For a more specific example; 4chan remains the only site to actually ban discussion of gamergate (and then discussion of the subsequent user blowback towards moot) due to it being a harassment campaign.
It has fairly bog-standard rules about not being a hateful jerk, it's just that enforcement of those rules is pretty spotty so the sites users consist mostly of hateful jerks.
For a more specific example; 4chan remains the only site to actually ban discussion of gamergate (and then discussion of the subsequent user blowback towards moot) due to it being a harassment campaign.