This book constitutes the refereed proceedings of the 4th International XML Database Symposium, XSym 2006, held in Seoul, Korea in September 2006 in conjunction with the International Conference on Very Large Data Bases, VLDB 2006. The 8 revised full papers were carefully reviewed and selected from 32 submissions. Their focus is on building XML repositories and covers XML query processing, caching, indexing and navigation support, structural matching, temporal XML, and XML updates. The papers are organized in topical sections on query evaluation and temporal XML, XPath and twigs, and XML updates