I've never been to the Virgin Islands but I know they catch a fair share of Marlin there. The Abacos in NE Bahamas have a big marlin tournament in April and May. Mexico is more known for their sailfish. For 4 days I might do a search for marlin charters in the Bahamas. Specifically Abaco. Freeport or Nassau might have too many tourist boats. You want a serious fisherman. Your short amount of time would make the Pacific coast of Mexico or Central America unrealistic. Bimini in the Bahamas might also be a great place to catch a marlin.