HDR support is the ICCmax version, not v4 as far as I'm aware. ICC v4 is not supported by ArgyllCMS (see link above), therefore not supported by Display Cal. The ICC workflow being printer-centric, HDR is irrelevant here. Anyway, HDR workflows moved to OCIO/ACES.

All I can say is ICC v2 is supported everywhere, many devs & projects didn't bother with v4, and ICCmax is supported nowhere and not planned for adoption for apps that need HDR. So, just stick to good old ICC v2, the future of ICC is 30 years ago.