Does not make sense. If it is not listed in the docs, it does not mean it will not work.
7.3 as not listed as well, but HDP runs perfectly on it...
Hortonworks is not officially supporting but if you want to install on 7.4 for learning purpose I'd say please go a head and give a try. It will work.
I'm running my cluster on 7.3 and didn't see any issues.
Yes, I have HDP running 7.3 with no problems as well.
Do not expect any problems from 7.4, but like to know official position
Yes, Officially Hortonworks is not supporting.
Here is the link but Sindhu from Hortonworks is already provided the link in her previous responce.