The only previous Mariner to hit four homers in the first four games of a season was Ken Griffey Jr. in 1997. Griffey went on to hit 56 that year and win the American League MVP.
"I'm just getting some good pitches to hit, trying to put the barrel on the ball and just let things happen," said Morse, who is off to the early lead among Major League home run hitters.
Maurer, 22, surrendered six runs and eight hits -- including home runs by Josh Reddick and Yoenis Cespedes -- to take the loss. He had no walks and one strikeout while throwing 74 pitches before turning the game over to Oliver Perez.
Maurer became the first pitcher to jump from Double-A to the Mariners rotation since Mike Hampton and John Cummings in 1993. He joins Michael Pineda (2011) as the only rookie to crack the season-opening starting five since 2000.
"He has good stuff, good demeanor out there and good presence," said manager Eric Wedge. "He's going to be fine. There were a couple mistakes he made, but you learn from those and big league hitters let you know."
Maurer gave up a two-run bomb to Reddick, the third batter he faced, when the A's right fielder ripped a 2-1 fastball well up into the seats in right. But the 6-foot-5 rookie settled in after that and retired nine in a row before Oakland got him for a run in the fifth on a Chris Young double and RBI single by John Jaso.
The A's tacked on three more runs in the sixth with a pair of doubles, followed by Cespedes' line shot over the left-field fence for his second homer of the series.
"It was definitely my first one," said Maurer, a former 23rd-round Draft pick out of Costa Mesa, Calif. "I was going out there trying to break off a little of the nerves and stress from just going out there the first time. I think there was a time there when I settled down, but then I started getting away from my fastball and getting behind in counts and got myself into trouble."
Veteran catcher Kelly Shoppach, making his first start for the Mariners, said Maurer handled himself well.
"He was really together," said Shoppach. "It seemed like he pitched with confidence. I'm sure he had a few pitches there in tough spots that he'd like to have back, but other than that he competed well. He didn't seem overwhelmed by what was happening. I just hope he remembered to enjoy it a little bit because this is the only time he's going to get to do it."
Maurer is just the 17th Mariners starter to make his Major League debut under the age of 23 as he leapfrogged other higher-rated prospects with an outstanding spring.
"That's quite an accomplishment," Shoppach said. "You work your whole life and dream about stuff like that. That might be more important than execution of pitches. We can't forget this is a game and a dream of ours to get to do every day. The other side of it is, we get better as we go.
"He can do a lot of things. There's a reason he made this team. He had some tough competition in Spring Training and proved he can pitch. We think he can pitch at this level and he will. He'll show us all how good he is as the season goes on."
The Mariners couldn't get much going early against A's right-hander A.J. Griffin, who is now 3-0 with a 2.12 ERA in three starts against Seattle since his midseason callup last year.
They pushed one run across in the fifth when Dustin Ackley scored on a sacrifice fly by Michael Saunders, then cut the lead to 3-2 with Morse's homer to left field in the sixth. But that was all the damage allowed by Griffin, who is now 8-1 in his young career.
Morse finished 2-for-4, his average now at .375 with six RBIs. Shoppach went 2-for-3 with a walk and Franklin Gutierrez hit safely for his fourth straight game and is off to a .333 start.
"It's a new ballclub," said Wedge. "We've got a lot of guys that are just now getting settled in. It takes a little time to get your feet underneath you and find your groove as a club, but we will."
The A's tacked on their final two runs in the eighth off reliever Lucas Luetge, with shortstop Jed Lowrie hitting Oakland's ninth double in the past two games. Of Oakland's 27 hits in the series, 16 went for extra bases with five home runs and 11 doubles.