Definition of big four:
(n) :
Any group of four major or largest corporations in a given industry, especially when such a small group has oligopoly or near-oligopoly.
(n) :
(finance, business, technology) The four largest corporations of Big Tech during the 2000s–2020s, usually defined as Alphabet (f.k.a. Google), Meta (f.k.a. Facebook), Amazon, and Apple.
(n) :
(business, accounting) The four largest global auditing firms, comprising Deloitte, Ernst & Young (EY), KPMG, and PwC.
(n) :
(finance) The four largest banks in any of various countries, or (internationally) the Federal Reserve, People's Bank of China, Bank of Japan, and European Central Bank.
(n) :
(fashion) The clothing trade shows (“fashion weeks”) in Paris, London, New York, and Milan.
(n) :
(music) A jazz rhythm derived from the marching band beat.