David Ortiz hit his 500th career double in the first and Jose Iglesias added an RBI single in the sixth for Boston.
Erlin was solid through the first three innings but could not get himself out of trouble in the fourth. Ortiz led off with a single and Mike Napoli followed with a walk. After striking out Jonny Gomes for the first out, Erlin walked Jarrod Saltalamacchia to load the bases for Snyder.
Snyder forced Erlin to throw nine pitches, fouling off five before hitting a double off the upper right corner of the Green Monster -- just a few feet more to the right and it would have been a grand slam. Snyder was thrown out trying to stretch out a triple, which would have left Erlin liable for another possible run.
Erlin was done for the night after Snyder tagged his 93rd pitch. He allowed three runs on five hits and three walks in 3 2-3 innings.
Iglesias added an RBI single in the sixth to put Boston up 4-0, then the Padres broke up Lackey's shutout bid on Guzman's homer to left in the seventh.
Boston nearly took the lead in the first when Dustin Pedroia walked and tried to score from first base on Ortiz' double down the right-field line. But Chris Denorfia hit second baseman Logan Forsythe with the relay and Forsythe threw out Pedroia at the plate.
NOTES: The fifth all-time series between the Padres and Red Sox continues Wednesday with RHP Edinson Volquez (6-6) starting for San Diego against Jon Lester (8-4). ... Ortiz became the 58th major leaguer to hit 500 doubles and the fifth active player to do so. ... Snyder was called up from Triple-A Pawtucket the week before and had two RBIs entering the game. ... Ellsbury extended his hitting streak to 12 games. ... Pedroia has reached base safely in 43 of 45 home games this season. ... San Diego SS Pedro Ciriaco returned to Fenway Park for the first time since being acquired by the Padres on June 14 for cash and got a quiet but warm round of applause in his first at-bat. Padres