This is the Fourth Periodic Report on the State of Acid Deposition in East Asia (PRSAD4), describing the outcomes of 20 years of EANET activities (2000-2019). The PRSAD4 is comprised of three parts; Part I: Regional Assessments, Part II: National Assessments, and Part III: Executive Summary. PRSAD4 Part I presents the main activities of EANET and provides an assessment of the state of acid deposition focusing on the special distribution, trend analysis, and, possibly, impact assessment of acid deposition and air pollution in the region. The data from the 2000-2019 period was also utilized for evaluating the current status of the environment in the EANET region, to share the results of the past 20 years provided by EANET in its regular phase, and to scientifically evaluate the monitoring data generated. It includes quality assurance (QA) and quality control (QC), impacts on ecosystems in East Asia, cross-cutting studies on the atmospheric environment in the EANET region, a summary, conclusions, and recommendations.