Freddie Mac ended 2012 with its single best quarterly showing since the company was placed into government conservatorship by the Federal Housing Finance Agency at the height of mortgage market implosion 4½ years ago. The GSE late this week posted fourth quarter net income of $4.5 billion. Compared to the third quarter’s earnings of $2.9 billion, profits grew by 55 percent, the company noted in its Securities and Exchange Commission filing.