This book contains the refereed proceedings of the Fourth Annual Mediterranean Ad Hoc Networking Workshop, Med-Hoc-Net 2005. After Sardinia (Italy), Mahdia (Tunisia), Bodrum (Turkey), the workshop took place this year on the beautiful island, fle de Porquerolles (France). The Med-Hoc-Net 2005 event consolidates the success of the previous editions of the workshop series. It aims to serve as a platform for researchers from academia, research, laboratories and industry from all over the world to share their ideas, views, results and experiences in the field of ad hoc networking. This year, 73 papers were submitted. We accepted 39 papers as full papers and 10 papers as short papers. Each full paper consists in 10 pages in these proceedings and was presented orally at the workshop whereas each short paper consists in 5 pages in these proceedings and was presented as a poster at the workshop. The selected papers were grouped according to the following topics: Physical and MAC layers, Power Consumption, Quality of Service, Routing, Connectivity, Optimization and Testbeds, Multicast and Broadcast, Clustering, Sensor networks, Auto-organization and Adaptation, Security and Evolution and finally Analysis. The accepted papers came from all over the world, mainly from diverse countries in Europe, but also from USA, Brazil, Tunisia, China and Taiiwan. We thank all authors for submitting their papers to Med-Hoc-Net 2005. For each paper, we provided two reviews (sometimes three reviews).