diff --git a/src/book.py b/src/book.py
index 74a0c37..0549089 100644
--- a/src/book.py
+++ b/src/book.py
@@ -29,8 +29,8 @@ class Book:
         "Verdana": "verdana,geneva,sans-serif"
     }
 
-    SUPPORTED_LEVELS = 3
-    SUPPORTED_HEADERS = {"h1", "h2", "h3"}
+    SUPPORTED_LEVELS = 4
+    SUPPORTED_HEADERS = {"h1", "h2", "h3", "h4"}
     HEADERS_LEVELS = {"h1", "h2", "h3", "h4", "h5", "h6", "h7", "h8", "h9"}