The new entity will not deal with financing currently offered by commercial financial institutions.
The four lenders are Japan Finance Corp. for Small and Medium Enterprises, National Life Finance Corp., Agriculture, Forestry and Fisheries Finance Corp. and the international finance division of the Japan Bank for International Cooperation.
The inauguration of the corporation marked a major overhaul of Japan's public finance sector.
The corporation "has a major role to play as the nation's only policy finance institution," Japanese Finance Minister Shoichi Nakagawa said at the inauguration ceremony.
It has 152 branches in Japan and 19 offices overseas with an initial workforce of 8,560 people.