Final answer:
Chile is the country that has the Juan Fernandez Islands where Alexander Selkirk was marooned for 4 years.
Step-by-step explanation:
The country that has the Juan Fernandez Islands and marooned the sailor Alexander Selkirk for 4 years is Chile. The islands became famous as the site where the mariner Alexander Selkirk was marooned in the early 18th century and are now a part of the South American country, Chile.
Selkirk's story is often believed to have inspired Daniel Defoe's novel, "Robinson Crusoe." Over time, countries like Spain and Portugal established treaties such as the Treaty of Zaragoza to divide up territories and navigate conflicts over newly discovered lands during the Age of Exploration.