H4, or HTML Heading 4, is a tag used in web development to define the fourth level of headings in a webpage. It is part of the HTML structure, which organizes content hierarchically. H4 tags are typically used for subheadings under H3 tags, helping to create a clear and logical flow of information. In terms of SEO, using H4 tags can improve the readability of a webpage and assist search engines in understanding the content's structure. Proper use of heading tags, including H4, contributes to better user experience and can enhance a site's visibility in search results.