Weird behaviour of column copy

Hello everybody
I have this code in workbook open

Private Sub Workbook_Open()
    With Sheet4
        .Visible = True
            With .Range("AH1", .Cells(.Rows.Count, "AH").End(xlUp))
                .Range("AM1").Resize(.Rows.Count, 1).Value = .Value
            End With
        .Visible = False
    End With
End Sub

The code is supposed to copy the contents of column AH to AM

The sheet4 is hidden .. the code would make it visible and after the copy of column AH it would be hidden
The weird behaviour I stated is that AM column have no data..
The data copied to column BT