Two Montgomery County brothers were sentenced to four years in prison today in the largest income tax evasion case ever brought in Maryland. Dominick and Joseph LaRosa of Potomac, also were ordered to pay $1.4 million in back taxes and penalties on $20 million in income from their coal-brokering business.

Maryland Attorney General Stephen H. Sachs said the outcome demonstrated the state's ability to detect and punish "sophisticated white-collar crime." Lawyers for the LaRosas said they would appeal.

Anne Arundel Circuit Judge Eugene M. Lerner, who found the brothers guilty during a trial in November, agreed to let them remain free on personal recognizance until the appeal is heard. They would be eligible for parole in a year, according to prosecutor Joseph L. Evans.

The LaRosa brothers' International Fuel Co. Inc. of Oxon Hill made a fortune from 1981 to 1985 buying coal in Pennsylvania and selling it to a West Virginia power plant. They acquired large houses, cars and investment portfolios and, eventually, drew the attention of state tax investigators, who accused them of defrauding the state of about $1.4 million in taxes.

Lawyers for Dominick LaRosa, 44, and Joseph LaRosa, 40, said the brothers' records are being challenged by federal tax authorities, who say they owe several million dollars. Federal authorities have frozen the LaRosas' assets and seized their automobiles. The state has obtained liens on their houses.

During their trial, the LaRosas said they had intended to pay their taxes using the "long-term completed contract" accounting method, under which they would pay their taxes when their five-year coal contract was over in 1985.