WASHINGTON – Fannie Mae is asking for a federal cash infusion of $15.3 billion after posting another big loss in the fourth quarter of last year. The mortgage finance company, seized by federal regulators in September 2008, lost $16.3 billion, or $2.87 a share in the October-December period. That takes into account $1.2 billion in dividends paid to the Treasury Department. It compares with a loss of $25.2 billion or $4.47 a share, in the year-ago period.