Selection from databound dropdownlist

I'm trying to get the value of the selection from the databound dropdownlist to be displayed in a textbox.
I have set the Autopostback property to true

However, the value that is displayed is always the first value from the database or rather the selected value is always the first value of the dropdownlist.

Here is the VB code:

Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
OleDbDataAdapter1.Fill(DataSet11)
DropDownList1.DataBind()

End Sub

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

End Sub

Private Sub DropDownList1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles DropDownList1.SelectedIndexChanged
Dim str As String

str = DropDownList1.SelectedItem.Text

TextBox4.Text = str

End Sub
End Class

Please Help. Any assistance will be very much appreciated