updating DIRECTORY.md

Author: AksharGoyal

DIRECTORY.md

@@ -936,6 +936,7 @@
     * [Sol1](project_euler/problem_009/sol1.py)
     * [Sol2](project_euler/problem_009/sol2.py)
     * [Sol3](project_euler/problem_009/sol3.py)
+    * [Sol4](project_euler/problem_009/sol4.py)
   * Problem 010
     * [Sol1](project_euler/problem_010/sol1.py)
     * [Sol2](project_euler/problem_010/sol2.py)