This document compiles the four foundational volumes of the Unified Substrate Theory (UST). The UST defines the quantum vacuum as a saturable relativistic elastic medium governed by a non-linear Dirac-Born-Infeld (DBI) action. By establishing a rigorous geometric isomorphism between the spatiotemporal elasticity of the vacuum and the dynamics of a 3-brane, this effective field theory successfully unifies microscopic and macroscopic observables. Volume 1 explains the topological nature and exact mass of the nucleon. Volume 2 and 3 detail the transition to high-density phenomenology, rigorously deriving a causal and thermodynamically stable Equation of State (EoS). Volume 4 presents the numerical resolution of the Tolman-Oppenheimer-Volkoff (TOV) equations, formally proving the existence of an ultra-compact stable branch (Twin Stars) capable of supporting the most massive pulsars observed (PSR J0740+6620), strictly constrained by the fundamental hadronic tension F = 266.8 MeV.