This book comprises the proceedings of the Fourth International Symposium on Beamed Energy Propulsion, offering the latest progress in the field of advanced space propulsion systems powered by laser, microwave, and charged beams, covering aspects from fundamental issues to future space exploration plans. Topics include: fluid dynamics in air-breathing laser propulsion; laser supported detonation and energy deposition; lightcraft; laser in-tube thruster; ablative propulsion and microthrusters; liquid propellant and more.