TOKYO—Just over a month after Japan's central bank vowed to reignite economic growth by flooding markets with yen, the currency fell to ¥100 to the dollar for the first time in four years, a milestone in efforts to end nearly two decades of economic stagnation.

The weaker yen's impact—the dollar has climbed 16% against the currency this year—is already trickling through the Japanese economy, pushing up prices of imported food and gas and drawing a flood of tourists whose currencies now buy more goods in Japan. It is bolstering sales and profit at exporters whose goods can be produced at lower prices for global markets. Early Friday in Tokyo, the dollar bought ¥101.12, compared with ¥100.60 late Thursday in New York and ¥99.02 late Wednesday.

The move past Y100 sent stocks up in Japan, with the Nikkei Stock Average rising 2.7% in early Friday trading. That caps a 68% boost since mid-November, including 18% during the past five weeks, since the Bank of Japan said on April 4 it would pump trillions of yen more into the markets each month.

More importantly, the yen's decline signals hopes for a more groundbreaking economic shift: the reversal of nearly two decades of stagnation, weak demand and declining prices. While Japanese officials insist they haven't been intentionally weakening the yen, its plunge is the direct result of the monetary policies promoted by new Prime Minister Shinzo Abe, who has declared it a central part of his "Abenomics" plan to beat deflation and boost growth.

In new signs of the impact of Abenomics, Japanese domestic institutional money started flowing overseas in pursuit of higher yields while bank lending rose at the fastest pace in four years, data released Friday showed.