Arizona will limp into September having lost seven of its past nine, including five of its past six against National League West rivals.
The series loss comes on the heels of getting swept by the last-place Padres in San Diego earlier in the week.
"Similar to San Diego, we felt like we had a good shot to go in there and win at least two out of three and we got swept, so you take nothing for granted," D-backs manager Bob Melvin said.
In both the San Diego and Los Angeles series, the pitching matchups heavily favored the D-backs. Both Dan Haren and Webb started a game in each series, and in all four games the D-backs lost.
"It's something that you don't expect, no doubt," outfielder Chris Young said. "It's tough to see Haren and Webby get beat, but it happened. The Dodgers came out and swung the bats well the entire series and [Derek] Lowe pitched a good game today. We'll move on past it."
It is the first time the club has lost four straight starts from their co-aces since May 27.
"That's why you play the game," Melvin said. "You look at the matchups and it's easy to say this team is a favorite over this team today, but sometimes it doesn't play out."
Right from the start of the game, Webb wasn't himself. After getting catcher Russell Martin to ground out to lead off the game, it was an uphill battle for the sinkerballer.
Dodgers outfielder Andre Either hit a solo home run in the next at-bat. After Webb walked Manny Ramirez, who went into the game 8-for-9 in the series, James Loney singled. After a Matt Kemp fielder's choice, third baseman Casey Blake blasted a 432-foot shot over the right-center-field wall, making the score 4-0 before the D-backs even batted.
In the third, after Kemp led off the inning with a single and advanced to second on a stolen base, he scored on Blake DeWitt's sacrifice fly.
In the fourth, Webb was replaced by new left-handed reliever Wil Ledezma after the Dodgers scored three more runs -- two unearned. Martin led off with a single and scored on Ethier's double. After Ramirez received his version of the Barry Bonds treatment with his second intentional walk, Loney struck out. Then D-backs third baseman Mark Reynolds let a ground ball go underneath his glove down the line at third as two runs scored.
Dodgers starter Derek Lowe (11-11) threw six shutout innings, allowing just four hits. The D-backs' only run came on a Reynolds solo home run -- his 26th of the season -- in the bottom of the seventh.
The D-backs saw 4 2/3 combined scoreless innings from relievers Ledezma, Max Scherzer and Chad Qualls.
The two teams will close out their season series next weekend with three games in Los Angeles.
"We've had a lot of tough losses, we've had some series that it looked like we were going to win, then we win the first game and then we don't," Melvin said. "We've also bounced back and responded at times this year. It's been a funny year all the way around for our division as a whole.
"If it ends up where we are right now, we'll all be happy no matter what the win-loss record looks like."