We have used complex conductivity measurements between 20 Hz and 1 MHz to probe the dielectric response in the low-temperature phase of the (2,5(OCH₃)₂DCNQI)₂Li organic conductor. The observed data reveal the existence of the relaxation process which is well described by a Debye-like expression for an overdamped response of a system with a single degree of freedom. Both the static dielectric constant (ε₀ = 3 × 10⁷) and the characteristic relaxation time (τ₀) are much larger than that expected for single-particle excitations. τ₀ is thermally activated in a manner similar to the dc conductivity. We attribute the observed relaxation to N = 4 charge density wave pinned by commensurability to the underlying lattice.