It is probably best to error trap each table to ensure that you are processing the correct tables e.g. as follows. I have assumed each table has six cells - change each as appropriate. However why do you need to change the tables? Wouldn't it be better to create your document from a template that already has the masthead correctly formatted?
Code:
Dim oTable As Table
Dim i As Integer
If ActiveDocument.Tables.Count > 3 Then
If ActiveDocument.Tables(1).Range.Cells.Count = 6 And _
ActiveDocument.Tables(2).Range.Cells.Count = 6 And _
ActiveDocument.Tables(3).Range.Cells.Count = 6 And _
ActiveDocument.Tables(4).Range.Cells.Count = 6 And _
ActiveDocument.Tables(4).Range.Information(wdActiveEndPageNumber) = 1 Then
For i = 1 To 4
Set oTable = ActiveDocument.Tables(i)
'Do something with otable e.g.
oTable.Cell(1, 1).Range.Text = "Test"
Next i
End If
End If