The stabbings were reported at about 2 a.m. at the hotel at 1770 S. Amphlett Blvd., San Mateo police Lt. Mike Brunicardi said.
Two of the stabbing victims, ages 19 and 20, are believed to be active gang members, and the other two victims, ages 17 and 20, recently dropped out of a gang, Brunicardi said.
All four were transported to separate hospitals and are expected to survive. Three had been released as of Monday afternoon, and the fourth remained hospitalized.
Brunicardi said the victims were attending several after-prom parties at the hotel, though he did not know if any of the victims were high school students or whether they were associated with any high schools.
He said no suspects have been arrested, and the San Mateo County District Attorney's Office is reviewing the case to determine whether any charges should be filed.
There are a number safety tips parents should discuss with their teens who plan to attend prom and after-prom parties, Brunicardi said.
Parents should talk about the dangers of drinking and driving, and discuss travel plans prior to the event. Teens should know that they can always call their parents if they need a ride in the middle of the night.
Police are still seeking witnesses to Sunday morning's stabbings, and encourage anyone with information to call the San Mateo Police Department at (650) 522-7700.