The Left() and Right() functions actually do the "unconcatenation". Wrapping their results in the Trim() function removes leading and trailing spaces from each substring. Since you have a year and week number, your probably don't have spaces and don't need to use Trim().
In design view, you can set a text box's Control Source to =Left([fieldname],4) and =Right([fieldname],2). Keep in mind, though, that they won't be updatable if you do this, because the control source is calculated. If you need these to be updatable, they should really be separate fields in the underlying table, but if that's not possible, you can use the Form_BeforeUpdate event procedure to re-concatenate the values of these fields and store them in the recordset field:
Private Sub Form_BeforeUpdate()
Me![fieldname] = Me![txtYear] & Me![txtWeek]
End Sub Rick Sprague