New Castle County police say 68-year-old Donald Grier succumbed to injuries he suffered back on December 3.
Investigators have also identified the other victims as 70-year-old Alicia Grier, 19-year-old Michael Santucci, and 53-year-old Linda Santucci.
They died the night of the incident.
Neighbors say the alleged assailant, 40-year-old Donald Grier, is the son of two of the victims.
Police arrested him following a car chase and crash in Summit Bridge.
Investigators have not yet revealed a motive.
Days after the deadly attack, dozens of neighbors gathered to honor the victims whose lives were cut short.
"It's how we get our peace out of such a terrible situation," said neighbor Jineen Flagg.