BERKELEY, Calif. -- For the second week in a row, fifth-ranked USC was less than dominant, again needing a fourth-quarter surge to stay unbeaten.

Last week, USC had to rally in the final minute to force overtime against Texas, eventually winning in the second overtime. On Saturday, California and USC were tied 13-13 entering the fourth quarter, but the Trojans took advantage of six turnovers, including four in the second half, to pull away for a 30-20 victory at Cal's Memorial Stadium.

"It's been a rough two games, but we're 2-0 in those games and that's all that matters to me," Trojans quarterback Sam Darnold said.

While winning its first road game of the season, USC (4-0, 2-0 Pac-12) won its 13th straight game overall, dating back to last season. It also beat Cal (3-1, 0-1) for the 14th consecutive time.

The pivotal play of the game occurred early in the fourth quarter when the ball slipped out the hand of Cal quarterback Ross Bowers as he was throwing a pass from just outside his end zone. The ball was caught in the air as a fumble recovery by USC's Uchenna Nwosu at the 3-yard line, leading to Stephen Carr's 1-yard scoring run that gave the Trojans a 23-13 lead.

Ykili Ross intercepted a Bowers pass on Cal's next possession, and he returned it 49 yards to the Cal 33, setting up Darnold's 4-yard scoring pass to Deontay Burnett that put the Trojans ahead 30-13 with 10:56 left.

"We were tight there until the fourth quarter," first-year Cal coach Justin Wilcox said, "and then there were a series of plays that went from 16-13, three down, and within a matter of minutes it was 30-13 and the end of the game.

"You can't commit six turnovers and expect to beat a team like that."

Cal outgained USC 416-356, but committing four more turnovers than the Trojans doomed the Bears.

"Outstanding play by the defense in the second half," USC coach Clay Helton said. "They played good in the first half, but they took it to another level in the second. To have 17 points created off six turnovers and hold them -- an offense that can put up points in the second half -- to only scoring on that last drive in the second half, credit the defense."

Darnold completed 26 of 38 passes for 223 yards and two scores, but he also threw his seventh interception of the season to go along with his nine TD passes. Freshman Carr, making his first start, was USC's leading rusher with 86 yards, and he added 47 yards on six receptions.

Bowers was 22 of 50 for 303 yards and a touchdown, but he threw four interceptions along with the critical fumble.

"This is going to be one that sticks with you," Bowers said. "Going to be hard to get over. I'm really disappointed in the way I played."

Patrick Laird added 82 yards on the ground for the Bears, and wide receiver Kanawai Noa caught six passes for 110 yards.

USC was without its leading rusher, Ronald Jones II, who averaged 107.3 yards per game over the first three games but was out with an ankle injury. Both teams were missing a starting wide receiver. Cal's Demetris Robertson missed his second straight game with an unspecified injury, and USC's Steven Mitchell was out with a groin injury.

Cal missed a chance to jump ahead early in the third quarter. Laird dropped what would have been a 14-yard touchdown pass, and Matt Anderson missed a 29-yard field goal attempt.

The Trojans scored the first points of the second half when Chase McGrath booted a 46-yard field goal early in the fourth quarter, giving USC a 16-13 lead. Nwosu then came up with the unusual fumble recovery that pushed the game in the Trojans' favor.

Cal had opened the scoring on Anderson's 36-yard field goal, but a Jack Jones interception of a tipped pass gave USC possession at the Cal 38, leading to a 37-yard field by goal by McGrath, which tied the game 3-3.

Cal took a 10-3 lead on Vic Enwere's 1-yard fourth-down run, but the Trojans tied the score 10-10 on Darnold's 16-yard touchdown pass to Tyler Petite.

The Trojans took a 13-10 lead on McGrath's 34-yard field goal, and Cal tied it on Anderson's 21-yard field goal on the final play of the half.

Although USC is 4-0, Helton realizes his offense needs work.

"The ball is being moved, that's not the issue," Helton said. "It's finishing drives."

NOTES: The last time Cal beat USC was 2003, when current Bears head coach Justin Wilcox was Cal's linebackers coach. ... Wilcox was USC's defensive coordinator in 2014 and 2015, and served under Clay Helton, who was the Trojans' interim head coach in 2015. Wilcox and three other assistants were fired six days after Helton was named USC's permanent head coach on Nov. 30, 2015. ... Saturday's contest was the Joe Roth Game, named after the Cal quarterback who led the Bears to a Pac-8 co-championship and a victory over USC in 1975 but died of cancer at the age of 21. Cal wore throw-back uniforms that the Bears wore in 1975. ... Most of the injured USC defenders who missed all or part of last week's game were available against Cal. Starting LB Porter Gustin (toe, biceps) did not play, however.