The term "Big Four" generally refers to the four major Allied powers during World War I and World War II: the United States, the United Kingdom, the Soviet Union, and France.
For World War I, the Big Four at the Paris Peace Conference in 1919 specifically represented the United Kingdom, United States, France, and Italy, but Italy's role was more limited than the others.
In either context, Germany, Japan, and other nations were not part of the Big Four. If you have a specific context or event in mind, please let me know!