The flat rotation curve of spiral galaxies is one of the core puzzles in modern astrophysics. Based on Newtonian gravity and the distribution of observed baryonic matter, the orbital speed of peripheral stars around galaxies is predicted to decrease following Kepler’s law as 𝑣 ∝ 𝑟−1 /2 . Nevertheless, multi-band neutral hydrogen radio and optical astrometric measurements consistently demonstrate nearly constant rotational velocities at large galactic radii. The mainstream ΛCDM cold dark matter paradigm postulates a pervasive dark matter halo surrounding every galaxy to compensate the missing gravity; however, decades of underground direct detection experiments worldwide have yielded entirely null results for dark matter particles. Meanwhile, the model suffers from inherent contradictions including the cusp-core problem of dwarf galaxies and the inability to naturally derive the empirical Tully-Fisher relation. The Modified Newtonian Dynamics (MOND) achieves satisfactory fitting on galactic scales but conflicts severely with observational constraints from CMB measurements and large-scale cosmic gravitational lensing. This work adopts the unified framework of Real-Virtual Dual-Field Theory (RVDT) with the identical exponential decay relation Λ(𝑟) = 𝑒−𝜉𝑟 , which shares the same physical origin as previous serial papers RVDT-01 (‘Oumuamua), RVDT-02 (C/2026 C1 comet) and RVDT-03 (Hubble Tension). Cosmic space consists of coupled Real Field and Virtual Field: the Real Field corresponds to all visible baryonic components including stars, interstellar gas and dust governed by standard gravitational law, while the omnipresent Virtual Field forms a background potential gradient. Dense baryonic Real Field in galactic central regions suppresses local Virtual Field contribution such that orbital motion is dominated by classical gravity; the sharp drop of matter density in galactic outskirts unlocks Virtual Field gradient effects to produce equivalent supplementary centripetal acceleration, filling the gravitational deficit and yielding flat outer rotation curves. Without introducing unobserved dark matter particles or revising fundamental gravitational laws, the RVDT model quantitatively reproduces hundreds of measured rotation curves from the SPARC dataset and naturally derives the Tully-Fisher empirical scaling law. It is fully consistent with CMB cosmologicalconstraints and prior RVDT predictions across Solar System and cosmic scales, with multiple falsifiable observational predictions available for verification via upcoming large-scale galaxy surveys and gravitational lensing observations.