The Red Sox have always been reluctant to sign aging players to lengthy deals, but after John Lackey received a 5-year contract, it's surprising to see them put such a limit on Beckett. One has to imagine the Sox have specific medical issues with Josh Beckett that make him less appealing than Lackey.
Beckett and the team have been talking about an extension over the course of the offseason, as Beckett is in the last year of his contract. It had seemed like the two sides were getting closer over March, but if the Sox are not willing to go to 5 years, one has to wonder if Beckett would really sign for only 4, especially given the Red Sox' willingness to give the older and recently injured Lackey 5 years.
Given the inclusion of injury protection in John Lackey's contract, it's possible the Red Sox could ask the same of Beckett if a deal were to go beyond 4 years.