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Confidential information has been omitted from this Exhibit and filed
separately with the Commission pursuant to a confidential treatment request
under Rule 24b-2. Exhibit 10.4 CONFIRMATION LETTER Kennecott #: 711Buyer:
    
    Otter Tail Power Company, a division
          of Otter Tail Corporation
NorthWestern Corporation,
           d/b/a NorthWestern Energy
Montana-Dakota Utilities Co., a division
           of MDU Resources, Inc.
c/o Otter Tail Power Company
As Agent for Big Stone Plant
P. O. Box 496
215 South Cascade Street
Fergus Falls, MN 56538-0496
    
    Seller:
    
    Kennecott Coal Sales Company
Caller Box 3009 (82717-3009)
505 South Gillette Avenue
Gillette, WY 82716Attn:
    
    Stacie Hebert
    
    Attn:
    
    Bruce MillerPhone:
    
    218-739-8635
    
    Phone:
    
    307-685-6114Fax:
    
    218-739-8629
    
    Fax:
    
    307-687-6009
 This Confirmation Letter (Confirmation) shall confirm the transaction
arranged October 31, 2003, between Otter Tail Power Company, a division of
Otter Tail Corporation (Otter Tail) and Kennecott Coal Sales Company
(Kennecott) pursuant to the Master Coal Purchase and Sale Agreement effective
June 1, 2004. The terms and conditions of this transaction are as follows: Kennecott to sell and deliver and Otter Tail to purchase and receive. Transaction Type:
    
    Physical CoalProduct:
    
    Sub-Bituminous coal; (*)Btu/Lb. and (*)Lbs. SO2/mmBtuBase Price:
    
    (*)per ton of coalShipment Period:
    
    January 1, 2005 through December 31, 2007
 Quantity: TonsDelivery Year
    
    (to the nearest unit train)2005
    
    
    (*
    )2006
    
    
    (*
    )2007
    
    
    (*
    )
 Delivery Point:
    
    FOB Railcar, Cordero Rojo Complex  Campbell County, WyomingTopsize:
    
    2 x 0 ASTM
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Quality:
    
    
    
    Coal Quality SpecificationsSection 9.03  Standard Btu and
Sulfur for price adjustments as
set forth below:
    
    Sections 6.02 & 6.04
Rejection Limits/
Non-Conforming ShipmentBtu/Lb
    
    (*)
    
    (*)Lbs. SO2/mmBtu
    (*)
    
    (*)
 Btu Adjustment:
    
    To reflect the actual heat content of the coal delivered, each month the Base Price of coal will be adjusted for any
variation from (*)Btu/Lb., using the following formula:
 Btu Adjustment Per Ton = P x
    
    (AR  BB)BB
 Where:P=
    
    The Base Price of coal per ton delivered during the month;AR=
    
    The monthly weighted average As-Received Btus per
pound of the respective coal[s] delivered to Otter Tail;
and,BB =
    
    The Base Btus per pound of the respective coal[s]
delivered to Otter Tail during the month; the BBvalue = (*)
 All shipment Btus and weighted average Btus shall be in
zero decimals. All prices for Btu adjustments shall be
calculated using floating-point decimals, with the result
being rounded to three decimal places as shown in the
following example:
 Sample info: P =(*)/ton, BB =(*), AR =(*) Example:Btu adjustment per ton
    
    = (*)= (*)= (*)= (*)
 Sulfur Adjustment:
    
    To reflect the actual sulfur content of Coal delivered, each month the Base Price of Coal will be adjusted
in accordance with the following formulas.For purposes of this adjustment, it shall be assumed that 100% of the sulfur in the Coal will be converted
to sulfur dioxide (SO2). The pounds SO2 per mmBtu shall be calculated in accordance with the following
formula based on Sellers lab analysis of the percent sulfur in the Coal and the calorific value of the
Coal:
 Lbs. SO2/mmBtu =
    
    Monthly Weighted Average Sulfur % in Coal X 20,000Monthly Weighted Average Btu/Lb.
 All shipment sulfur percent and weighted average sulfur
percent shall be stated in two decimals. SO2 for the period
billed shall be calculated using floating-point decimals,
with the result being rounded to two decimal places as shown
in the following example:Sample info: Monthly Weighted Average Sulfur % in Coal =(*),
Monthly Weighted Average Btu/Lb. =(*)Example:Lbs. SO2/mmBtu = ((*)X 20,000) / (*)= (*)= (*)
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Sulfur adjustment in $/ton of Coal = (Base Lb. SO2/mmBtu  Actual Lb. SO2/mmBtu) X Actual Btu/Lb. X $ADI1,000,000ADI =The SO2 Monthly Average Price
published by Air Dailyfor the month preceding
delivery.Base Lb. SO2/mmBtu = (*)All shipment SO2 and weighted average SO2 shall be stated in
two decimals. All prices for sulfur adjustments are to be
calculated using floating-point decimals, with the result
being rounded to three decimal places as shown in the
following example:Sample info: Actual Btu =(*), Base SO2 =(*), Actual SO2 =(*),SO2 Allowance (ADI) =(*)
 Example:Sulfur Adjustment in $/ton of Coal
    
    =((*) (*)) X (*)X $(*)) / 1,000,000= $(*)= $(*)
 IN NO EVENT SHALL EITHER PARTY BE LIABLE TO THE OTHER PARTY FOR, AND EACH OF
THE PARTIES WAIVES THE RIGHT TO SEEK INCIDENTAL, CONSEQUENTIAL, OR PUNITIVE
DAMAGES UNDER THIS AGREEMENT. Please confirm that the terms and conditions stated herein accurately reflect
your understanding of our agreement by signing and returning to Leslie Thorn at
Sellers address. By:
    
    /s/ Ward Uggerud
    
    Date:
    
    June 22, 2004Otter Tail Power Company, a divisionof Otter Tail CorporationBy:
    
    /s/ Andrea Stomberg
    
    Date:
    
    June 29, 2004Montana-Dakota Utilities Co., a divisionof MDU Resources, Inc.By:
    
    /s/ Michael J. Hanson
    
    Date:
    
    July 14, 2004NorthWestern Corporation,doing business as NorthWesternEnergyBy:
    
    /s/ Kelly A. Cosgrove
    
    Date:
    
    June 1, 2004Kennecott Coal Sales Company
 (*) Confidential information has been omitted and filed separately with the
Commission pursuant to Rule 24b-2. 3