Changed three major types to four major types  in nodes.py
diff --git a/jinja2/nodes.py b/jinja2/nodes.py
index f9da1da..c827e2b 100644
--- a/jinja2/nodes.py
+++ b/jinja2/nodes.py
@@ -104,7 +104,7 @@
 
 class Node(object):
     """Baseclass for all Jinja2 nodes.  There are a number of nodes available
-    of different types.  There are three major types:
+    of different types.  There are four major types:
 
     -   :class:`Stmt`: statements
     -   :class:`Expr`: expressions