Auto-Number for PO spreadsheet

Perhaps this:-

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
If Target.Address(0, 0) = "A4" Then
    Target.EntireRow.Insert
    Target.Offset(-1).Interior.ColorIndex = xlNone
    Target.Offset(-1) = IIf(Target = "", 2016000, Target + 1)
End If
End Sub