A site can have IPv6 itself and still load its checkout, captcha, chat or fonts from hosts that only have IPv4. This walk loads dj-zook.com's home page and checks every outside host it pulls from.
What this does and doesn't show
Phones on IPv6-only networks (many mobile carriers) usually still reach IPv4-only hosts through a translator their carrier runs (NAT64 / 464XLAT). "IPv4 only" here means that host depends on that translator, not that it's broken today.
Only the home page's HTML is read. Scripts that load more scripts later (tag managers, widgets) aren't followed, and pages behind a sign-in aren't seen.
Before calling a host "IPv4 only" we ask two DNS resolvers twice more. Some names are split across CDNs, so one answer has IPv6 and the next doesn't; those are marked "IPv6 sometimes".
For hosts with IPv6 we also try one connection over IPv6. "IPv6, didn't answer" means that connection failed from our server.
Some sites answer automated visitors with a bot check instead of their page; when that happens we say so and show nothing.