SEATTLE -- With two of the American League's best home run-hitting teams as well as sluggers like Nelson Cruz, Chris Davis and Mark Trumbo at Safeco Field this week, it's no wonder that the long ball played a big factor in the four-game series between the Seattle Mariners and Baltimore Orioles.

What was surprising is that the bat of Seattle Mariners outfielder Seth Smith ended up stealing the show.

Smith homered for the fourth game in a row Sunday, when his first career grand slam led the Mariners to a 9-4 win to complete the four-gram sweep over the Baltimore Orioles.

"It's big," Smith said of winning four in a row against the American League East leaders. "Any sweep's big, and to come out and beat a really good team like that shows what we're capable of."

Smith hit his 10th home run of the season, and his fourth of the series, to lead a Seattle offense that had 10 hits. The Mariners (43-39) won their fourth in a row while the Orioles were swept in a four-game series for the first time this season.

Baltimore (47-34) matched its longest losing streak of the season, having lost four in a row. The Orioles got tagged for 45 hits and 31 runs during the four-game sweep, with 26 hits and 21 runs coming in the losses Saturday and Sunday. Baltimore's starters were responsible for 21 earned runs over 18 innings in the series -- an ERA of 10.50.

"It's been a challenge for us all year," manager Buck Showalter said of the Orioles' starting pitching issues. "We've got to get that corrected if we're going to get where we want to go."

Ubaldo Jimenez (5-8) was the latest Baltimore starter to struggle, allowing six runs off six hits over 4 1/3 innings Sunday.

Smith's grand slam gave the Mariners a 4-0 lead in the bottom of the third inning. Smith has homered in four games for the first time of his career, having hit one in every game of the series. He also became the seventh Seattle player this season to hit 10 home runs.

"It's fun," Smith said. "All the homers are great, but to hit a grand slam is even better."

Baltimore responded with a three-run top of the fourth, led by Mark Trumbo's 24th home run of the season. Trumbo's solo shot cut the Seattle lead to 4-1, then the Orioles patched together three more hits in a row. J.J. Hardy's two-run double brought Baltimore to within 4-3.

Ketel Marte's RBI double in the bottom of the fourth put the Mariners ahead 5-3, then a balk on Baltimore's Jimenez brought in another run for a 6-3 Seattle lead.

The Orioles had a chance to get it all back in the top of the seventh, which began with leadoff hitter Adam Jones singling to left to chase Seattle starter Hisashi Iwakuma from the game. Hyun Soo Kim followed with a slap-hit single to left against a defensive shift, bringing up the heart of the Orioles' order with runners on first and third. Seattle rookie Edwin Diaz struck out Manny Machado and Chris Davis before Trumbo hit an 0-2 pitch to left, scoring Jones to cut the Seattle lead to 6-4. Diaz then struck out Jonathan Schoop, stranding two runners on base.

Seattle responded with a three-run seventh, highlighted by three consecutive doubles. Adam Lind followed Kyle Seager's two-run double to center field with an RBI double to left as the Mariners opened up a 9-4 lead.

Iwakuma (8-6) allowed four runs off 10 hits over six innings in his first game against Baltimore since throwing a no-hitter against the Orioles last August.

Seattle's Smith went 8-for-17 with 11 RBIs during the four-game series. His Sunday grand slam was the first of his nine-year career.

The Mariners earned their first four-game sweep since 2012 with the win, while moving within 8 1/2 games of first-place Texas in the American League West race. Seattle moved into a second-place tie with the Houston Astros, who host the Mariners in a three-game series that begins Monday.

The division-leading Orioles lost another game in the AL East standings with the loss. Boston beat the Angels on Sunday to move within three games of Baltimore.

"It's time to put our best foot forward," said Showalter, whose team begins a three-game road series against the Los Angeles Dodgers on Monday. "We have to put this behind us and get ready for Los Angeles."

NOTES: The pitching-strapped Orioles optioned Saturday starter Tyler Wilson to Triple-A Norfolk on Sunday morning and called up left-handed reliever Ariel Miranda, who was at Safeco Field for the game after being summoned from Norfolk late Saturday night. Miranda made his major league debut a few hours after arriving on an early-morning flight from the East Coast and ended up pitching two innings. He allowed three runs off four hits over two innings. ... To make room for Miranda on the 40-man roster, the Orioles transferred LHP Brian Duensing from the 15- to the 60-day disabled list. ... Seattle 1B Dae-Ho Lee was not in the lineup Sunday for the first time since June 21. The Mariners had 1B Adam Lind and his left-handed bat in the lineup to face Baltimore's right-handed starter, Ubaldo Jimenez. The Mariners went into the season platooning Lee and Lind but had both their bats in the lineup seven times in the 10 games leading up to Sunday. ... Orioles C Caleb Joseph was in the lineup Sunday for the first time since coming off the disabled list following testicular surgery. Joseph took a foul tip to the groin on May 30 and hadn't played in a major league game again before Sunday.