This report is the Fourth Periodic Report on the State of Acid Deposition in East Asia (PRSAD4). The EANET data of 2015-2019 is utilized for evaluating the current status and impact of the acid deposition and air pollution in the EANET region. This report also includes the long-term analysis of the temporal trend and variation, spatial distribution, and possible impact on the eco-system of acid deposition and air pollution in East Asia based on the twenty-year monitoring data from 2000 to 2019. Other national and international monitoring results were also used as references for additional assessment for interpretation of EANET data.