Abstract We propose Four-Sector Cosmology (FSC), a theoretical framework in whichthe observable universe (Sector I) is one of four CPT-symmetric sectors arising froma single quantum fluctuation at t = 0. The four sectors are defined by the signs ofspatial coordinates (x) and temporal direction (t): Sector Coordinates Identification Sector I (+x, +t) Baryonic matter — our observable universe Sector II (−x, +t) Dark energy candidate (w ≈−1 Sector III (+x, −t) Dark matter candidate (time-reversed relic) Sector IV (−x, −t) CPT-conjugate (white-hole source; now mostly decayed) the observable universe (Sector I) is one of four CPT-symmetric sectors arising froma single quantum fluctuation at t = 0. The four sectors are defined by the signs ofspatial coordinates (x) and temporal direction (t): The initial state satisfies exact CPT symmetry: ΩI,0 = ΩII,0 = ΩIII,0 = ΩIV,0 =25%. Starting from this single postulate, FSC simultaneously resolves four majoropen problems in modern cosmology: 1. Baryogenesis: Sector IV decay satisfies all three Sakharov conditions, gen- erating ηb = 6.1 ×10−10, consistent with BBN observations. 2. Dark matter: Sector III survives as a stable relic with ΩIII(t0) = 0.268, matching Planck 2018. 3. Dark energy: Sector II accumulates Sector IV decay energy, growing from 25% to ΩII(t0) = 68.3% — the first derivation of ΩΛ from fundamental sym- metry principles without fine-tuning. 4. Hubble tension: Five-component decomposition yields ∆HFSC0 = 2.69 km/s/Mpc, consistent with the observed 2.6 ±0.7 km/s/Mpc discrepancy. The framework makes five falsifiable predictions testable with DESI DR2, Euclid,Rubin-LSST, LZ, and gravitational-wave standard sirens within the next five years.