The 4th International Workshop on Community Networks and Bottom-up-Broadband(CNBuB 2015) will be held in August 24-26, 2015 in Rome, Italy, co-located with the 3rd FiCloud 2015. Community networking, together with the Bottom-up-Broadband initiative, is an emerging model for the Future Internet across Europe and beyond where communities of citizens build, operate and own open IP-based networks and services, a key infrastructure for individual and collective digital participation. The CNBuB 2015 workshop aims at serving as a meeting point for researchers and practitioners, both from academia and community networks, to discuss state-of-the-art and in-progress research, exchange ideas and share experiences. The workshop will address theoretical and practical challenges of community networks and bottom-up-broadband, including insights from other disciplines that contribute to the socio-technical-economic understanding of the community network operation and growth. To support the growth of community networking and bottom-up-broadband in society, it is necessary to improve the state-of-the-art in design, operation, and evolution of community networks. Towards this objective, the research community has the opportunity to contribute with more sustainable, adaptive, scalable, integrated, autonomic solutions.