You hold in your hands the proceedings of ESAS 2007, the Fourth European Workshop on Security and Privacy in Ad hoc and Sensor Networks. The wo- nd rd shop took place in Cambridge, UK, on the 2 and 3 of July 2007. The workshop was European in name and location but it was definitely transatlantic in scope. We had a program chair from Europe and one from the USA, and membership of our program committee was almost evenly split - tween those two regions. When looking at participation, the workshop was even more global than that: the submitted papers came from 25 countries in 6 con- nents. We received 87 submissions. After quick-rejecting 5 papers deemed to be out of scope, the remaining 82 papers were each reviewed by at least three PC members. The two program chairs, who did not submit any works, had sole authority to decide which papers to accept and reject, based only on the directive that quality had to be the primary criterion, in order to form a proceedings volume of high international relevance. The number of papers to be accepted was not set in advance: it was selected a posteriori so as to include only solid, innovative and insightful papers. The resulting acceptance rate of about 20%, very strict for a workshop, is a testimonial of how selective we chose to be in accepting only high quality papers.