The Big 4 are all accounting firms in origin but have expanded into other areas in recent years. They are best known for their audit work which is required for companies over a certain size in order to verify their financial statements and provide assurances to investors and tax authorities that the company is complying with their obligations.
In the early 2000s, the Big 4 expanded their services into other professional services such as consulting and law but following controversy over the independence of firms providing advisory services to clients, they audited PwC, EY, and KPMG all divested these services. Deloitte was the only firm to retain its consulting practice.
However, since 2010 consulting has been the primary driver of growth for these firms. They all now offer audit, tax, risk assurance, consulting, and transaction services to their clients. For PwC, the biggest tax advisor in the world, consulting now generates as much revenue as their tax practice.
EY, Deloitte, and PwC have also all acquired strategy consulting firms meaning that they now compete directly with the Big 3.