Short answers, pulled from the story.
The Allies were formally established as a named group on the 1st of January 1942, when 26 countries signed the Declaration by United Nations. The United States, the United Kingdom, the Soviet Union, and China signed first, followed by representatives of 22 other allied countries the next day.
The Big Four were the United Kingdom, the United States, the Soviet Union, and China. The Declaration by United Nations officially recognized them as the "Four Powers," and they were also referred to as the "Four Policemen" of the United Nations.
The United States officially joined the Allies after Japan attacked American and British territories on the 7th and the 8th of December 1941. On the 8th of December, the US Congress declared war on Japan at the request of President Franklin D. Roosevelt, and Germany and Italy declared war on the United States on the 11th of December.
Total Lend-Lease aid amounted to $42 to $50 billion. The British Commonwealth and empire obtained about half of that total. The United States separately supplied the Soviet Union with $10 billion worth of aid, equivalent to approximately 7 percent of Soviet war production.
The Declaration by United Nations was a formal agreement signed on the 1st of January 1942 by 26 countries pledging to oppose the Axis powers and not negotiate a separate peace. Countries that subsequently adopted the declaration were considered Allied members; the French provisional government signed on the 26th of December 1944, and eleven more nations adhered in early 1945.
China had been at war with Japan since the Marco Polo Bridge incident of the 7th of July 1937 and formally declared war on the Axis on the 9th of December 1941. China committed more troops than any member outside the Big Three; more than 1.5 million Japanese troops were tied down in the China theatre, troops that otherwise could have been deployed elsewhere.