"We only had seven 4-stars in WWII with 12 million in the military. "
12, I think, but most of them were created in 1945 after the promotions of Marshall, Eisenhower, Arnold, and MacArthur.
The expansion of the 4-star rank REALLY ratcheted up with the world-wide duties the US took on in the years after World War II, especially with NATO and SEATO commands.