From 2004 to 2007, the top five U.S. investment banks each significantly increased their financial leverage (see diagram), which increased their vulnerability to a financial shock. Changes in capital requirements, intended to keep U.S. banks competitive with their European counterparts, allowed lower risk weightings for AAA securities. The shift from first-loss tranches to AAA tranches was seen by regulators as a risk reduction that compensated the higher leverage. These five institutions reported over $4.1 trillion in debt for fiscal year 2007, about 30% of USA nominal GDP for 2007. Lehman Brothers went bankrupt and was liquidated, Bear Stearns and Merrill Lynch were sold at fire-sale prices, and Goldman Sachs and Morgan Stanley became commercial banks, subjecting themselves to more stringent regulation. With the exception of Lehman, these companies required or received government support. Lehman reported that it had been in talks with Bank of America and Barclays for the company's possible sale. However, both Barclays and Bank of America ultimately declined to purchase the entire company.

How many U.S. investment banks significantly increased their financial leverage from 2004 to 2007?