The 49-year-old gunman entered the apartment below his at around 2am and opened fire, hitting one victim with five bullets, a second with three and a third with two.
The three men, aged 44, 32 and 42, are in a serious condition.
A 40-year-old woman was hit by two bullets but was not seriously injured, police said.
One of the victims was found inside the apartment, while the others had managed to flee, with the gunman chasing after them into the corridor and the street.
Two other people at the party managed to escape without injury.
The gunman, described by neighbours as a loner, surrendered inside his apartment without resisting to police.