While Reddit has the obvious place for this topic, I have no idea, where to start similar discussions on 4chan.
Should I do it at all? Or is it a rather bad idea?
/tg/ seems to be suitable only for D&D campaign settings.
While the comments identifying 4chan as an unsavory place are well-founded, I don't think it's true that unsavory places have nothing to offer the intrepid world-builder.
/tg/ certainly has a gaming world bias, but it's also populated by sundry sci-fi and medieval fantasy nerds who often provide feedback that is broadly applicable to the task of worldbuilding.
There is also a literature board, where any literature except fanfiction can be discussed. Presumably "fanfiction" does not cover workshopping original worlds. This is the closest I can find to the old text only board they used to have for writers, back when I was an active user.
The various topical boards may also be of use to you for particular questions, especially if your primary interest is in fringe theories. The Paranormal, Science & Math, and History & Humanities boards are likely suspects for fruitful discussions, particularly if you find your questions getting "serioused out" of SE sites.