The length of DB mangas differs. It depends on factors like the story's complexity and the author's vision. Some might run for just a few volumes, while others could have dozens.
In my view, 'fiction db' is probably a term used in a particular field or industry related to fiction. It could be a database used by authors, publishers, or researchers to organize and access data about fictional works. But again, we'd need more details to know for sure.
As a fan of web novels, I don't have the latest information about iPartment 4. However, according to my knowledge, iPartment 4 is a TV series, and the plot has not been publicly announced yet. If I have more information, I will try my best to answer your questions.