These are not HTML4 tags (elements), you can confirm this from the Index of Elements in the HTML 4.01 Specification.
There is reference to them as "HTML Snippets" in the book Introducing .NET 4.0: With Visual Studio 2010 by Alex Mackey. The book describes them as "doctype snippets" (shortcuts) for the following:
- html4f - HTML 4.01 Frameset doctype declaration
- html4s - HTML 4.01 Strict doctype declaration
- html4t - HTML 4.01 Transitional doctype declaration
So, on the assumption that you are using these "snippets" in this context, that is as a shortcut for document type declaration, then you can refer to the section HTML version information in the HTML 4.01 Specification for guidance on when to use them.