Screen readers do indeed have a feature that allows the users to browse just the headings. As others have mentioned, it's likely flagged as an accessibility is

Screen readers do indeed have a feature that allows the users to browse just the headings. As others have mentioned, it's likely flagged as an accessibility issue since having an h4 follow an h2 indicates a broken document structure. It's also probably included in the tool because things like that usually (though not always) mean that the developer is just using the h4 tag based on how it looks as opposed to actual semantic meaning, and the tool is designed to get you to put more thought into what you're actually building.