Big 4 is a short abbreviation for 4 of the audit companies who are dominating the supply of auditing services worldwide. Big 4, namely Price Waterhouse Coopers, Ernst & Young, Deloitte and KPMG have their offices across the different regions in the world.
The services provided by Big 4 could be categorized into three main areas: advisory, taxation & audit. There would be different sub-categories within each area. For instance, there are financial service audit, commercial audit (including audit across different industry, e.g. manufacturing), small to medium enterprise audit, US GAAP audit, and others. Each specialization would have a target group (by industry, by region, by accounting standard).
Recruitment processes in Big 4 are considered competitive and selective. Since their main focus are corporate client, including the big conglomerate group, they prefer high fliers in the University.