VB.NET Generate a random string of numbers and letters

Private Sub btn_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btn.Click
        Dim s As String = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
        Dim r As New Random
        Dim sb As New StringBuilder
        For i As Integer = 1 To 4
            Dim idx As Integer = r.Next(0, 35)
            sb.Append(s.Substring(idx, 1))
        Next
        Label2.Text = sb.ToString
    End Sub