Officers responded to a report of gunshots at about 1:30 p.m. and could still hear shots being fired from the house when they arrived, police spokesman Sean Whitcomb said.
Police said that while they were arriving a man in his 50s ran into the house. Two more shots were fired and he ran back out, telling officers his wife had shot herself.
When police were able to go inside, they found the four dead, including a woman in her 50s they believe is the shooter, Whitcomb said.
Whitcomb said police don't believe anyone else is involved and were not looking for anyone.
In addition to the older woman, the dead are two women in their late teens and a man in his 30s, police said. Their relationships were not immediately known.
Assistant Police Chief Jim Pugel told Seattlepi.com a 42-year-old wounded woman - who fled the house and is expected to survive - was able to speak to officers when they arrived.
"All she said was, "My mom has gone crazy,"' Pugel said.
Police said two handguns were recovered from the house, which is in a lower middle-class neighborhood a block north of southwest Seattle's city limits. No motive for the shootings was known, officers said.
Neighbor Bobby Miller told KOMO that police showed up almost immediately after the shots were fired.
"I just heard gun shots. I come out and cops were all over," Miller said. "Just 'bam, bam' and that was it. Then screaming over here."