How to make default value for combobox with selectedvalue=0

Problem

Selected value to any item in combo box give me null why

Details

I work in windows form c# visual studio 2015

And i get data of combo box from excel sheet 2007

excel sheet have two column

MemberId represent value to combo box

MemberName represent text to combo box

var fileName = string.Format("{0}\\Book3105", Directory.GetCurrentDirectory());  
        var connection = string.Format("Provider=Microsoft.ACE.OLEDB.12.0;" + "Data Source={0};Mode=ReadWrite;Extended Properties=Excel 12.0 Xml;", fileName);  
  
        OleDbConnection con = new OleDbConnection("Provider=Microsoft.ACE.OLEDB.12.0;" + "Data Source=" + "D:\\Book3105.xlsx" + ";" + "Extended Properties=\"Excel 12.0;HDR=YES\"");  
        OleDbConnection con = new OleDbConnection(connection);  
  
        try  
        {  
            con.Open();  
            str = "select * from [sheet2$]";  
            com = new OleDbCommand(str, con);  
            oledbda = new OleDbDataAdapter(com);  
            ds = new DataSet();  
            oledbda.Fill(ds, "[sheet2$]");  
            con.Close();  
  
            dt = ds.Tables["[sheet2$]"];  
            int i = 0;  
  
  
            for (i = 0; i <= dt.Rows.Count - 1; i++)  
            {  
                comboBox4.Items.Insert(0, "select member");  
                comboBox4.SelectedIndex = 0;  
                comboBox4.Items.Add(dt.Rows[i].ItemArray[1]);  
            }  
        }  
        catch (Exception ex)  
        {  
            MessageBox.Show(ex.Message);  
        }  

When debug this line under button give me null

and message box show to me when select any item in combo box .

private void buttontest_Click(object sender, EventArgs e)  
        {  
  
           if(Convert.ToInt32(comboBox4.SelectedValue)==0)  
            {  
                MessageBox.Show("wrong value");  
  
            }  
  
        }