Reflection - Exception By Setting A Field Value

I didn't work a lot with Reflection and now I'm having troubles by writing a method to set a field value of an unknown type.
 
I wrote a little example program to show, where I cannot find a solution. I explain the problem in the following points:

- there is a class with 4 fields, which 2 are of type primitive int and 2 are Integer
- at line 27 is set the name of the field I want to modify
    - if is set field1 or field2 (of type int) there is no problem. In this case the 'case "int"' of the following switch is executed
    - if is set field3 or field4 (of type Integer) is executed the 'default' case of the following switch and at line 56 is thrown the cast exception that "Cannot cast java.lang.String to java.lang.Integer"
 
import java.lang.reflect.Field;
 
public class TestClass {
    public int field1 = 1;
    public int field2 = 2;
    public Integer field3 = new Integer(3);
    public Integer field4 = new Integer("4");
  
 [Code] .....