It looks like starting with 4.0.0, C and C++ libraries got split in separate codebases and their version numbers have decoupled -- the latest msgpack-c release is 4.0.0, while C++ had 4.0.0, 4.0.1, 4.0.2, 4.0.3 and 4.1.0 releases[1].
[1] https://github.com/msgpack/msgpack-c/releases
