Private Sub Worksheet_Change(ByVal Target As Range)
On Error Resume Next
If Target.Validation.InCellDropdown And Target.Column = 4 Then
If Err = 0 And Target <> vbNullString Then
Old = Target.Value
Application.EnableEvents = False
Application.Undo
If InStr(Target, Old) = 0 Then Target = Old & vbLf & Target
Application.EnableEvents = True
End If
End If
End Sub