We present extensive multi-frequency VLA and VLBA observations of the radio-bright supernova (SN) IIb SN 2004C that span ∼40-2793 days post-explosion. We interpret the temporal evolution of the radio spectral energy distribution (SED) in the context of synchrotron self-absorbed (SSA) emission from the explosion's forward shock as it expands in the circumstellar medium (CSM) previously sculpted by the mass-loss history of the stellar progenitor. VLBA observations and modeling of the VLA data point to a blastwave with average velocity ∼ 0.06c that carries an energy of ≈10 49 erg. Our modeling further reveals a flat CSM density profile ρ CSM ∝R −0.03 ± 0.22 up to a break radius R br ≈(1.96 ± 0.10) × 10 16 cm, with a steep density gradient following ρ CSM ∝R −2.3 ± 0.5 at larger radii. We infer that the flat part of the density profile corresponds to a CSM shell with mass ∼0.021 M , and that the progenitor's effective mass-loss rate varied with time over the range (50 − 500) × 10 −5 M yr −1 for an adopted wind velocity v w =1000 km s −1 and shock microphysical parameters e = 0.1, B = 0.01. These results add to the mounting observational evidence for departures from the traditional single-wind mass-loss scenarios in evolved, massive stars in the centuries leading up to core collapse. Potentially viable scenarios include mass loss powered by gravity waves and/or interaction with a binary companion.