Exxon Mobil (XOM.N) plans to sell a large part of its 50 percent stake in TonenGeneral Sekiyu KK (5012.T) back to its Japanese refining partner in a deal that could be worth about 300 billion yen ($3.9 billion), and will make an announcement as early as Monday, four sources with direct knowledge of the matter said.

Exxon Mobil will retain about a 20 percent stake in TonenGeneral but the deal will mark a de facto retreat from the world's third-largest economy by the U.S. oil giant, which is focusing its resources on emerging markets and development of natural resources.

The move could also spark realignment among Japan's oil refiners, which have been cutting capacity to cope with falling demand caused by a weak economy and a shift to more efficient and environmentally friendly forms of energy, analysts have said.

Reuters reported earlier this month that Exxon was in talks to sell part of the stake back to TonenGeneral.

TonenGeneral, which imports and distributes Exxon oil in Japan, ranks as the country's No. 2 refiner behind JX Holdings (5020.T). Smaller rivals include Idemitsu Kosan Co (5019.T), Cosmo Oil (5007.T) and Showa Shell (5002.T).

Exxon and TonenGeneral aim to complete the deal around summer, the sources told Reuters on condition of anonymity.

TonenGeneral will seek funds from Sumitomo Mitsui Banking Corp, Sumitomo Trust Banking, Bank of Tokyo Mitsubishi UFJ and Mitsubishi Trust Bank to buy back the stake, the sources said.