Mac High Sierra file system block size
No, that is not extremely large. The command "stat -f %k ." does not as such give you the block size on disk, but rather it is a guide from the OS to user programs as to how large amounts of data at a time it should transfer to/from disk for optimal performance.

With modern SSDs it is not uncommon for that value to be 4 MB as you indicate, instead of the 4 kB that was a kind of "default value" for many years, and not really observed by user space programs anyway.