The problem is also found in the development (fhi-*) font packages, except it is even worse. In the fhi packages, both C4 and C5 fonts are called C4, O3 and O4 and O5 fonts don't have suffixes in font names, both S4 and S5 fonts are called S4. All font names should include proper suffixes.
Speaking of suffixes, what are the suffixes supposed to represent? The 4's and 5's only slightly differs in weight and the change is only visible without antialiasing, but the character widths are the same. The O fonts have dotted zero while C and S fonts do not, but is that the only difference? In any case, the differences should be documented.
When viewing the fonts in Windows, some fonts have wrong or inconsistent font names:
Ume P Gothic O4 (ume-pgo4.ttf) -> Ume P Gothic
Ume P Mincho O3 (ume-pmo3.ttf) -> Ume P Mincho
Ume Gothic O4 (ume-tgo4.ttf) -> Ume Gothic
Ume Mincho O3 (ume-tmo3.ttf) -> Ume Mincho
Ume UI Gothic O4 (ume-ugo4.ttf) -> Ume UI Gothic
The problem is also found in the development (fhi-*) font packages, except it is even worse. In the fhi packages, both C4 and C5 fonts are called C4, O3 and O4 and O5 fonts don't have suffixes in font names, both S4 and S5 fonts are called S4. All font names should include proper suffixes.
Speaking of suffixes, what are the suffixes supposed to represent? The 4's and 5's only slightly differs in weight and the change is only visible without antialiasing, but the character widths are the same. The O fonts have dotted zero while C and S fonts do not, but is that the only difference? In any case, the differences should be documented.