Big Four (or Big Five, Big Six, Big Eight) The largest accounting firms in the United States as measured by revenue.
Investopedia Says: Most of the auditing for public companies is done by these firms. This term is constantly changing largely due to mergers. At one point the grouping was called the Big Eight, then Big Six, Big Five, etc.
Related Terms: Accounting Audit Auditor's Report Big Three Generally Accepted Accounting Principles - GAAP