This Practice Note provides details on the key features of the Energy Company Obligation (ECO) obligation period 4 (ECO4). It provides details on what ECO is and the legislation underpinning ECO4, the Electricity and Gas (Energy Company Obligation) Order 2022, SI 2022/875. It also focuses on who has obligations under ECO4, what the relevant thresholds are and how the Home Heating Cost Reduction Obligation (HHCRO) works. There is also information on transfers of qualifying actions and obligations, ECO brokerage, surplus action, Ofgem’s role, monitoring and compliance, as well as coverage of recent developments, including the launch of the Great British Insulation Scheme.