This volume constitutes the selected papers of the fourth internationalconference on Metadata and Semantic Research, MTSR 2010, held in Alcaláde Henares, Spain, in October 2010.The 28 full papers presented together with 3 invited lectures werecarefully reviewed and selected from numerous submissions. The papersfocus on theoretical and practical aspects of metadata research in abroad sense: models, languages, standards, applications, etc. and alsoon the application of Semantic Web technologies or any other kind ofadvanced metadata expression language to Web applications. They cover awide range, from formal papers to empirical studies, including differentkind of application and technology reports as well as user studies.