Criteria in a Query

I am trying to use a combo box in a form to select the criteria for my query.  It works for filtering if you do choose a value in the combo box.  However, if the combo box is left blank, I want the query to show all records for that field.  However, It will not show any fields that are blank. This is not a required field, so I still want all the fields to show if there is no value in the combo box.  Here is my equation: 

=IIf(IsNull([Forms]![Combined Board Report Form].[Combo4]),[Medium 1],[Forms]![Combined Board Report Form].[Combo4])

Thanks.