How to call function on selected text of dropdownlist in asp.net

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ASP.NET

how to call function on this code

DropDownList4.SelectedItem.Text = ?

I have one registration form. I have made cascading dropdownlist in which data of state dropdownlist is loaded on selected index change of Country. sql database is having table Country and State.

I am loading all data(other page, like here it is edit profile) which was submitted during registration on page load event at respective textboxes and dropdownlist. The problem is that Country is loading perfectly on country dropdown but the value of respective state is not loading on state drop down.

code that i am using is as below

protected void loadinternshipdata()
       {
           SqlConnection con = new SqlConnection("Data Source=ja-pc\\sqlexpress;Initial Catalog=Internship;Integrated Security=True;Pooling=False");

           con.Open();
           SqlCommand cmd3 = new SqlCommand("select * from Internship where CompanyID=" + Convert.ToInt16(Session["CompanyID"].ToString()), con);

           SqlDataAdapter daa = new SqlDataAdapter(cmd3);
           DataTable dtt = new DataTable();
           daa.Fill(dtt);
           if (dtt.Rows.Count > 0)
           {

               DropDownList3.SelectedItem.Text = dtt.Rows[0]["Country"].ToString();
              DropDownList4.SelectedItem.Text = dtt.Rows[0]["State"].ToString();
           }
       }