This text constitues the refereed proceedings of the Second International Symposium on Agent Systems and Applications and the Fourth International Symposium on Mobile Agents, ASA/MA 2000 held in Zurich, Switzerland in September 2000. The 20 revised full papers presented were carefully reviewed and selected from 107 submissions. The papers are organized in topical sections on migration, security issues, systems and applications, mobile agent applications, applications of multi-agent systems, communication and mobility control, co-operation and interaction.