The former World Series MVP and Cy Young Award runner-up said he did not press for a fifth year like the Red Sox gave free agent John Lackey because he was happy to stay with a team that will be competitive.
"This is a special place," Beckett said Monday at Fenway Park news conference, a day after pitching the season opener against the New York Yankees. "I think everybody that's had a chance to play here knows that."
Beckett gets a $5 million signing bonus, payable when the contract is approved by the commissioner's office, and annual salaries of $15.75 million.
The MVP of the 2003 World Series for Florida and a key part of Boston's 2007 title, Beckett allowed five runs and eight hits in 4 2-3 innings on Sunday night in a 9-7 victory over the New York Yankees.
By announcing the deal after the season started, Beckett's extension isn't included in luxury tax calculations until next year. If the Red Sox announced it before the opener, it would have raised his average annual pay in this year's calculations, potentially increasing the team's 2010 luxury tax.
The extension gives the Red Sox a long-term look at one of baseball's best rotations.
Lackey signed for five years and $82.5 million over the offseason. Jon Lester and Clay Buchholz won't be eligible for free agency until the 2014 season ends. Daisuke Matsuzaka, who is starting the season on the disabled list, is under contract through 2012.
"It's hard to be an elite organization without starting pitching," Red Sox general manager Theo Epstein said. "When you have that pitching, you don't want to let it get away. It's hard to acquire in free agency and it's hard to acquire in trades. This is the building block."
The 29-year-old Beckett had sought a five-year deal comparable to Lackey's, but the Red Sox were thought to be concerned about Beckett's right shoulder. Instead, he gets a modest bump over his teammate, and a big raise from the $12.1 million he is earning this season in the option year of his previous deal.
"A lot of people look at what you lost. I look at what I gain," he said. "I look at four more years of stability, knowing I'm going to be on a winning team. The season gets old when you're losing 90 games. I know I'm going to have a chance to win every year here."
Beckett was 12-10 with a 4.03 ERA in 27 starts in 2008, when he went on the disabled list twice. He missed the start of the season with a lower back strain then was sidelined in late August and early September with inflammation in his right elbow, but last year he had a 17-6 record and 3.86 ERA in 32 starts.
Epstein said there were no medical concerns.
"We have outstanding medical reports," he said. "The commitment we make today demonstrates that. There's not a medical reason why (his success) shouldn't continue."
Beckett has a 106-68 record with a 3.81 ERA and 1,331 strikeouts in his career. In 14 career postseason starts, he is 7-3 with a 3.07 ERA and three shutouts -- one behind Christy Mathweson for the most in major-league history.
He went 20-7 with a 3.28 ERA and made his first All-Star team in 2007, finishing second in the AL Cy Young voting to CC Sabathia, the left-hander Beckett faced Sunday night in the first game of the major league season. In the last five seasons, only Sabathia, with 82 wins, and Roy Halladay, with 81, have more than Beckett's 80.
In 2003, his second season in the majors, Beckett pitched a 2-0 complete game over the Yankees in Game 6 as the Marlins clinched the World Series. In 2007, he pitched the opening game of Boston's Series sweep over Colorado, allowing one run in seven innings.
Beckett went 15-8 with a 3.38 ERA for the Marlins in 2005, but he was traded to Boston in the offseason along with third baseman and 2007 World Series MVP Mike Lowell when the Marlins needed to lower their payroll.