This book contains the proceedings of the Fourth Meeting on CPT and Lorentz Symmetry, held at Indiana University in Bloomington on August 8-11, 2007. The Meeting focused on experimental tests of these fundamental symmetries and on important theoretical issues, including scenarios for possible relativity violations.Experimental subjects covered include: astrophysical observations, clock-comparison measurements, cosmological birefringence, electromagnetic resonant cavities, gravitational tests, matter interferometry, muon behavior, neutrino oscillations, oscillations and decays of neutral mesons, particle-antiparticle comparisons, post-Newtonian gravity, space-based missions, spectroscopy of hydrogen and antihydrogen, and spin-polarized matter. Theoretical topics covered include: physical effects at the level of the Standard Model, General Relativity, and beyond; the possible origins and mechanisms for Lorentz and CPT violations; and associated issues in field theory, particle physics, gravity, and string theory. Contributors consist of the leading experts in this very active research field.Contents:Improved Tests of Lorentz and CPT Symmetry using Noble-Gas Masers (A Glenday et al.)Rotating Experiments to Test Lorentz Invariance in the Photon Sector (M E Tobar et al.)Perspectives on Lorentz and CPT Violation (V A Kostelecký)Lorentz Violation in a Diffeomorphism-Invariant Theory (R Jackiw)Studies of CPT Symmetry with ASACUSA (R S Hayano)Torsion Balance Test of Preferred-Frame and Weak Coupling to Polarized Electrons (B R Heckel et al.)Seeking a Solution of the Pioneer Anomaly (M M Nieto & J D Anderson)Preliminary Results from a Test of CPT and Lorentz Symmetry using a K-3He Co-magnetometer (T W Kornack et al.)Ives-Stilwell for the New Millennium (M A Hohensee et al.)Data Tables for Lorentz and CPT Violation (V A Kostelecký & N Russell)and other papersReadership: Theoretical and experimental physicists with interests in relativity, spacetime symmetries, and underlying unified theories.