In a world of self-published manga/anime/other related stuff, there are doujin, doujinshi, doujinshika and doujinka. What's the difference between those four?
Also, are these term used exclusively for anime and manga related?
A Doujin, actually stands for a group of people that stand to achieve something, or share the same interests/hobbies.
However, it also depicts the work they make. Which in Western cultures, is often perceived to be derivatives of existing work, or analogous to fan fiction. Which is often true, as works are based upon popular games/manga/anime, but is not limited to. As there also exists original works within the Doujin.
The term Doujinshi stands for self published work. It is often considered a broader category of Doujin, as it covers Anime, hentai, games but also art collections. Where as other sub categories of Doujin, such as Doujin music/soft/game/H cover far more specific areas.
Doujinshika / Doujinka
Both these terms stand for creators of Doujin. These terms are often used interchangeably in both english and Japanese a like.
The usage of Doujinshika is said to be so limited that is often even considered to be incorrect. So the generally preferred term is Doujinka
Doujinka often reffer to themselves as sākuru or in case of single artist kojin sākuru. Which in place, reffers to the original meaning of Doujin,
a group of people that stand to achieve something, or share the same interests/hobbies.
Well, no. These terms strike a larger category than just A&M and it's related contents, and could be used out of it's context all together.
However, they are so often used in context of A&M and it's related works, that especially in Western Cultures they will be considered A&M related.
Ka in Dojinshi-ka have two corresponding Japanese. I'm not sure which one you talk. I want to describe literal meaning.