0.2 and 0.4 are different "major releases" of rust crates as you say. The major release is determined by the first non-0 component in the version number. The issue is that debian appears to only allow one version even if there are multiple major versions.

If debian is fine with packaging versions 2.0 and 4.0 but not 0.2 and 0.4, then debian does not understand rust version numbers.