SAN FRANCISCO (AP) — Jayson Werth hit his first homer in almost a month and drove in three runs, and the Washington Nationals capitalized on Matt Cain’s erratic start to beat the San Francisco Giants 6-2 on Wednesday night for their fourth straight win.Cain (1-4) walked the first three batters he faced before Adam LaRoche’s two-run single highlighted a three-run first inning. Werth’s solo shot in the fifth and two-run single in the ninth provided the rest of the pop for the Nationals, who have won 10 of 12, including the last three at San Francisco.Tanner Roark (5-4) allowed two runs and seven hits in six-plus innings for Washington. He struck out four and walked none.Brandon Crawford and Pablo Sandoval each drove in a run for the Giants, who have lost a season-high tying three straight after winning five in a row to take over baseball’s best record.Cain’s struggles might have been the most disappointing thing of all for the Giants.In his second start since a stint on the disabled list with a strained right hamstring, the former ace reverted back to his early season woes against Washington.