The Big 4 refers to the four largest global accounting and professional services firms: Deloitte, PwC (PricewaterhouseCoopers), EY (Ernst & Young), and KPMG. They primarily provide audit and assurance services (checking companies’ financial statements), tax services (helping with tax planning and compliance), and consulting/advisory services (advising businesses on strategy, technology, risk, and operations). They work with large corporations, governments, and organizations around the world.