- A hurricane warning is also in effect in Barbados, St Vincent and the Grenadines, and Tobago.
- Meteorologists say that it is unusual for a hurricane of this strength to form this early in the year.
Hurricane Beryl has made landfall on Carriacou Island in Grenada, according to the US National Hurricane Center (NHC).
The hurricane, which has regained strength in the past hours, has sparked warnings of life-threatening winds and dangerous storm surges.
A hurricane warning is also in effect in Barbados, St Vincent and the Grenadines, and Tobago.
Earlier, airports and businesses shut down and residents across the Caribbean were urged to seek shelter as the potentially devastating storm hit the region.
Dozens of flights were cancelled across the region as Beryl approached on Sunday night, while leaders urged the public to heed warnings.
"It is not a joke," said the prime minister of St Vincent and the Grenadines, Ralph Gonsalves, reminding people of the devastation caused by past hurricanes in the Caribbean.
In a national address from his official residence, Mr Gonsalves said he was seeking shelter in his basement.
"The roof, certainly the old part of the roof, may not survive winds at 150mph (241 km/h). I am making preparations to go downstairs," he said.
Beryl's strength has been fluctuating.
The hurricane was upgraded to a category four on Monday after slightly weakening earlier.
The NHC said fluctuations in strength were likely to continue but warned that portions of the Windward islands should prepare for "potentially catastrophic wind damage".
It said St Vincent and the Grenadines as well as Grenada were at the highest risk of damage.
Hurricane shelters opened at 18:00 local time (22:00 GMT) on Sunday.
The prime minister of Barbados, Mia Mottley, also urged citizens to be alert.
"We need to be ready. You and I know when these things happen, it is better to plan for the worst and pray for the best," she said.
"Do not let your guard down," she added.
Meteorologists say that it is unusual for a hurricane of this strength to form this early in the year.
"Only five major (Category 3+) hurricanes have been recorded in the Atlantic before the first week of July," hurricane expert Michael Lowry posted on X, formerly known as twitter.
"Beryl would be the sixth and earliest this far east in the tropical Atlantic," Mr Lowry wrote.
The US National Oceanic and Atmospheric Administration (NOAA) has warned that the North Atlantic could get as many as seven major hurricanes this year - up from the average three in a season.
It said record high sea surface temperatures were partly to blame.
Meteorologists have also remarked on how quickly Beryl developed.
The storm strengthened from a tropical depression into a major hurricane - category three or above - in only 42 hours, hurricane expert Sam Lillo told Associated Press news agency.
Much of the region has heeded the warnings.
Shops have closed and people have stocked up on fuel and groceries.
Grenada issued a state of emergency and St Lucia imposed a "national shutdown", ordering schools and businesses to remain closed.