SAN JUAN, Puerto Rico — Hurricane Beryl strengthened into what experts called an "extremely dangerous" Category 4 storm as it approaches the southeast Caribbean, which began shutting down Sunday amid urgent pleas from government officials for people to take shelter. Hurricane warnings were in effect for Barbados, St. Lucia, Grenada, and St. Vincent and the Grenadines. Beryl's center is expected to pass about 112 kilometers south of Barbados on Monday morning, said Sabu Best, director of Barbados' meteorological service. "This is a very serious situation developing for the Windward Islands," warned the National Hurricane Center in Miami, which said that Beryl was "forecast to bring life-threatening winds and storm surge." Beryl was located about 570 kilometers east-southeast of Barbados. It had maximum sustained winds of 215 kph and was moving west…