The currency was trading just over the 100 mark to a U.S. dollar early Friday in Asia after crossing that level the previous day in New York.
A lower yen makes Japanese goods cheaper and thus helps boost its exports. The export-dependent Japanese economy has been trapped in chronic deflation for years and the government and the Japanese central bank have introduced numerous stimulus measures for an economic recovery. Japan's economy is the third largest in the world after that of the United States and China.
CNNMoney said the yen's decline to a four-year low indicated investor confidence in the steps taken by the Bank of Japan as well optimism about the U.S. economy.
The report quoted Kathy Lien, managing director at BK Asset Management, as saying: "We're seeing a broad based dollar rally. You can attribute the overall trend to BOJ policies, but this immediate move is more about the dollar."
Lien said the yen could dip to 105 against the dollar as central banks in many countries cut interest rates.
In the United States, the job market showed strength as the number of Americans filing for jobless benefits fell to a five-year low last week.
South Korea, the fourth largest economy in Asia, Thursday joined the European Central Bank and other central banks in cutting its benchmark interest rates, the first such cut in seven months.
"All of this creates demand for dollars," Lien told CNNMoney.