Four of five former Bernard Madoff workers convicted of aiding his $17.5 billion fraud have escaped with sentences much lighter than prosecutors sought, as a judge placed most of the blame for the scheme on its mastermind, Bloomberg News reported yesterday. The prison terms meted out by U.S. District Judge Laura Taylor Swain at hearings this week moved some defendants and their lawyers to tears. Another attorney thanked her profusely for a sentence far less than even he suggested. Madoff’s former teenage secretary, who went on to run the investment-advisory business at the center of the scam, got six years behind bars, less than half what the U.S. wanted. She was followed by former computer programmers Jerome O’Hara and George Perez, who got 2 1/2-year terms instead of the recommended eight. And Daniel Bonventre, who ran Madoff’s broker-dealer unit for four decades, got 10 years, less than half the time sought by prosecutors, who have spent six years on the case.