Four is bcrypt's minimum cost (work factor), built into the algorithm itself by the original authors (Provos and Mazières) in 1999.
The original/reference implementation is OpenBSD's, which forces the cost to 4 if a cost less than 4 is requested.
The original selection of 4 was likely because a goal Provos' local testing during development was for hashing to be sufficiently slow, but not "too" slow (this is a reply to my question from Provos himself).
So while could compile a local version of the code yourself and override this minimum, I do not recommend it (for anything other that for curiosity / testing purposes). bcrypt cost 4 is much too slow on modern CPUs (compared to 25 years ago!). I would consider cost 10 to be the minimum today for secure production use, and cost 12 if you can get away with it.