Add h4 formatting for markdown formatter

src/MDFormatter.php

@@ -77,6 +77,19 @@ final class MDFormatter {
 
     }
 
+    /**
+     * Applies markdown formatting for fourth level headline.
+     *
+     * @param string $input Headline text.
+     *
+     * @return string
+     */
+    public static function formatMarkdownHeadline4(string $input):string {
+
+        return '#### ' . $input . PHP_EOL;
+
+    }
+
     /**
      * Returns a horizontal rule in markdown.
      *