Update 'ZipCodeFInalExam_Cohort4.md'

more unit test.

ZipCodeFInalExam_Cohort4.md

@@ -1,5 +1,5 @@
 # The Test
-Instructions: Read each question carefully. Answer all questions.
+Instructions: Read each question carefully. Answer all questions. Any software must have exhaustive unit test.
 
 Time limit: 2 hours. Begin immediately.