arg buffer problem(Graphics.DrawMeshInstancedIndirect) in compute shader?

Hi all. why do we need to define args array with 5 values? if I define with 4 elements I can not see anything!

Code (CSharp):

1. public class CClass{
2. int[] args={0,0,0,0,0};
3. void Start(){
4. args[0]=mesh.GetIndexCount();
5. args[1]=numInstance;
6. argBuffer.setData(args);
7. }
8. }