This book constitutes the refereed proceedings of the 4th International XML Database Symposium, XSym 2006, held in conjunction with the International Conference on Very Large Data Bases, VLDB 2006. The book presents 8 revised full papers, focused on building XML repositories and covering query processing, caching, indexing and navigation support, structural matching, temporal XML, and XML updates. Topical sections include query evaluation and temporal XML, XPath and twigs, and XML updates.