I found the first 2 levels of c4 very useful to get a quick sense what the system consists of, what it interacts with and who the users are. Also because it is something non-tech people can understand and reason about.

But for level 3, 4 it gets tedious to do the diagrams at time and harder to keep them updated over time. Personally I prefer to just read the code and only have diagrams in special cases at this point