Bank of America Corp. announced today that it agreed to acquire Countrywide Financial Corp. in an all-stock transaction worth approximately $4 billion, making B of A the nation’s largest mortgage lender and loan servicer.

The purchase constitutes another major but risky acquisition for Bank of America Chairman and CEO Kenneth Lewis, who has spent more than $100 billion through mergers to create the second-largest U.S. bank, Reuters reported. Countrywide had $408 billion in mortgage originations in 2007 and has a servicing portfolio of about 9 million loans totaling $1.5 trillion. The purchase also includes Countrywide’s Lender Placed insurance and other businesses.

“Countrywide presents a rare opportunity for Bank of America to add what we believe is the best domestic mortgage platform at an attractive price and to affirm our position as the nation’s premier lender to consumers,” Lewis said in a release. “Countrywide customers will gain access to a broad set of consumer products including credit cards and deposit services. Home ownership is a fundamental pillar of the U.S. economy and over time it will be a key area of growth for Bank of America.

“We are aware of the issues within the housing and mortgage industries,” Lewis continued. “The transaction reflects those challenges. Mortgages will continue to be an important relationship product, and we now will have an opportunity to better serve our customers and to enhance future profitability.”

Countrywide shares decreased 82 cents, or 10.5 percent, to $6.93 in pre-market trading, after climbing 51.4 percent yesterday in anticipation of the transaction; Bank of America increased 75 cents to $40.05 in pre-market trading, Reuters reported. Under the terms agreement, Countrywide shareholders would receive 0.1822 of a Bank of America share for each of their shares. The transaction values Countrywide at $7.16 per share, a 7.6 percent discount to the Thursday closing price.