I just replaced some of fs2 usage with fs4 and got following error when calling file.allocate() on macOS from multiple users:

No space left on device (os error 28)

In one example it happened with ~750GB file and then file.allocate() was called with the same file size again, which failed with ^ error.

Here is what implementation looks in fs2, it is much more verbose than in fs4:
https://github.com/danburkert/fs2-rs/blob/9a340454a8292df025de368fc4b310bb736f382f/src/unix.rs#L108-L137

I'll be switching to fs2 for that use case for now.