It happened at a house on Cowden and Bright in the Beachwood area of Merced County.
Deputies found two people with gunshot wounds. They were rushed to the hospital. Two others who had been shot showed up at the hospital on their own.
Police say 19-year-old Mateo Soriano died at the hospital.
Two of the victims were minors.
Detectives believe the party got out of hand and there may have been uninvited guests, which led to someone opening fire.
Deputies say there were several people who left the party before they arrived -- they're working to track them down.
Later Sunday morning, at around 10:15, there was a shooting in Atwater.
Shots fired on Winton Way near Fir Avenue sent one person to the hospital in an ambulance. A second gunshot victim showed up at the hospital on their own.
"It's tough. This is a major thoroughfare, people are concerned for their safety," said Atwater Police Lt. William Novetzke. "Right now everything is preliminary. We feel for the families of this incidents and we're currently investigating. As soon as we know more, we'll inform the public."
It's unclear what led up to the shooting.
Anyone with information about either shooting, should call detectives.