Section 6. All out-lots, valued by said commissioners at more than forty dollars per acre, shall be rated number one; all out- lots, valued at more than twenty and less than forty dollars per acre, shall be rated number two; all out-lots valued at more than ten and less than twenty dollars per acre, shall be rated number three; and all out-lots, valued at less than ten dollars, or at ten dollars per acre, shall be rated number four; Provided, That in making such valuation the value of buildings shall not be included.