add week 4 questions

Week 4.md

+## Week 4
+### Problem 1
+
+Write a Java program to find the common elements between two arrays (string values)
+
+```
+Array1 : [Python, JAVA, PHP, C#, C++, SQL]                                                                    
+Array2 : [MySQL, SQL, SQLite, Oracle, PostgreSQL, DB2, JAVA]                                                  
+Common element is : [JAVA, SQL]
+```
+
+### Problem 2
+
+Write a Java program to find the second largest element in an array.
+
+```
+Original numeric array : [10789, 2035, 1899, 1456, 2013, 1458, 2458, 1254, 1472, 2365, 1456, 2165, 1457, 2456]
+Second largest value: 2458
+```