This is the fourth and last volume of the invaluable publication At the Frontier of Particle Physics: Handbook of QCD. In this volume the reader will find three important sections. The first is devoted to QCD-based phenomenology. It covers issues deeply woven into the fabric of particle physics: weak interactions of light quarks (J Bijnens) and heavy quarkonium physics (A Hoang). The second section is a report on recent advances in the understanding of confinement in three dimensions in the Georgi-Glashow model (I Kogan and A Kovner). The third section deals with lattice QCD (A Kronfeld) and loop equations (A Dubin and Yu Makeenko).The five reviews in Volume 4, together with the 33 reviews in Volumes 1-3, constitute a full encyclopedia, covering all aspects of quantum chromodynamics as we know it today. The articles have been written by recognized experts in this field. Combining features of a handbook and a textbook, this is the most comprehensive source of information on the current status of QCD. It is intended for students as well as physicists — both theorists and experimentalists.