The Federal Trade Commission said yesterday a $4 million fine--the highest civil penalty ever imposed for violating an FTC order--was levied against an Oregon lumber company for failing to sell a fiberboard plant that it had agreed to dispose of by March 1981.

The company, Louisiana-Pacific Corp. of Portland, attacked Tuesday's decision by U.S. District Judge James Redden of Portland as "unreasonable" and said it is considering an appeal.

But Redden said he wanted the fine to be large enough to deter Louisiana-Pacific and others from considering "modest" penalties for failing to comply with FTC orders "as an acceptable cost of doing business."

The FTC said that Louisiana-Pacific, the nation's second largest producer of particleboard, had illegally continued to operate its Rocklin, Calif., plant for 20 months after it had agreed to get rid of it.

The company obtained the plant when it purchased Fibreboard Co. in 1978 but was ordered to sell the facility because the FTC said the purchase substantially reduced competition in the fiber and particleboard business.

Those products are made by pressing together wood shavings and scraps -- ordinarily waste materials -- and are used in the manufacture of furniture, doors, shelves and building materials.

The FTC in March 1979 ordered Louisiana-Pacific to sell the plant to an approved buyer within two years and the company agreed to comply with the order.

Louisiana-Pacific now appears to have a buyer for the plant. It asked the FTC last week to approve the sale of the Rocklin plant to Roseburg Lumber Co. for $15 million. Under the terms of that deal, which the FTC said would not affect the fine, Roseburg would buy the plant's land and machinery but Louisiana-Pacific would continue supplying the wood shavings and scrap wood to make the fiberboard and would provide marketing aid for export sales.

Louisiana-Pacific Vice President John Hart said the company has tried to ease the FTC's concern of reduced competition in the industry by voluntarily cutting its capacity to provide particleboard and medium density fiberboard by more than 200 million square feet a year. The Rocklin plant's capacity is only 75 million square feet a year.