Ferocious Category 4 hurricane Gustav roared into the Gulf of Mexico on Sunday after wreathing havoc in Cuba, as the U.S. National Hurricane Center warned Americans to expect the worst in the coming hours.Forecasters said Gustav was just short of Category 5 strength when it made landfall Saturday on mainland Cuba near the community of Los Palacios in Pinar del Rio. Having left at least 81 people dead in the Dominican Republic, Haiti and Jamaica, Gustav smashed first across Cuba's Isle of Youth, with more than 200,000 residents, and then tore across mainland Cuba southwest of Havana, which has a population of more than two million people. "Gustav is forecast to remain a major hurricane through landfall along the northern Gulf coast," the U.S. National Hurricane Center said.