Fixed HW4 problem

Author: thomas mcandrew

_toc.yml

@@ -17,7 +17,7 @@ chapters:
 - file: Homework01.ipynb
 - file: Homework02.ipynb
 - file: Homework03.ipynb
-- file: Homework04.ipynb
+- file: Homework4.ipynb
 #- file: Homework06.ipynb
 #- file: Homework07.ipynb
 #- file: Homework08.ipynb