Thousands of people had already fled their homes when Hurricane Ida hit the Louisiana coast with wind speeds of 240 km / h.
By noon, over 100,000 families in Louisiana were without electricity.
Long lines formed at gas stations on Saturday as people rushed to leave the coast. Traffic jams have formed on Highway 10 from New Orleans.
The US National Hurricane Center defines a Category 4 hurricane as a hurricane that will cause "catastrophic damage" causing "serious damage" to homes, breaking trees and cutting power lines, causing power outages that can last for months and leaving the area uninhabitable for "weeks or months".
Arriving at a barometric pressure of 930 millibars, Ida became the fifth strongest hurricane to hit the United States, judging by the speed of the wind.
In terms of central pressure, it ranks ninth among the strongest hurricanes to land in the United States.