According to Press TV, the US National Hurricane Center downgraded the storm after winds decreased to 125 mph (201 km) from maximum sustained winds of 130 mph (209 km).
Harvey is the strongest storm to make landfall in the US in over a decade and the strongest to hit Texas in over 50 years.
The last Category 4 storm to hit the US was Charley in 2004 in Florida, while the last Category 4 storm to hit Texas was Carla in 1961.
Fueled by warm Gulf of Mexico waters, Harvey grew from a Category 1 early in the Friday morning to a Category 4 by the evening.
“The tropical storm force winds have already commenced on the Gulf Coast,” said National Hurricane Center Spokesman and Meteorologist Dennis Feltgen. “You’ve essentially run out of time for outdoors preparations. You need to find a safe place and you need to stay there.”
States of emergencies have been declared for 30 counties in Texas and statewide for Louisiana and so far thousands have fled the areas. However, many have defied mandatory evacuation orders, stocking up on food, fuel and sandbags.
Texas governor Greg Abbott wrote a letter to President Donald Trump, requesting federal aid in order to be able to cope with the storm.
“The storm surge, coupled with the deluge of rain, could easily lead to billions of dollars of property damage and almost certainly loss of life,” he said in his letter.
Trump tweeted: “I am closely watching the path and doings of Hurricane Harvey … BE SAFE!”
White House Press Secretary Sarah Huckabee Sanders said Friday, “The President’s highest priority is the safety of the public and of first responders. Those who ignore evacuation orders could be putting both themselves and first responders in danger.”