This year marks the fourth year of the International Working Conference on Active Networks; this volume contains the papers accepted for full presentation at the conference. These proceedings are proof that Active Networks is a vibrant ?eldwithtrulyinternationalparticipation, withresearchdonenotonlyinthe contextofformalprograms(suchasFAINandDARPAActiveNets), butalso in a number of independent research programs. We received signi?cantly more submissions than could possibly be accom- datedduringthetwodaysofsessions, andcombinedwiththedesireforhigh standards this has resulted in a very selective...