We present new observations, carried out with IRAM NOEMA, of the atomic neutral carbon transitions [C I](³P₁–³P₀) at 492 GHz and [C I](³P₂–³P₁) at 809 GHz of GN20, a well-studied star-bursting galaxy at z = 4.05. The high luminosity line ratio [C I](³P₂–³P₁) /[C I](³P₁–³P₀) implies an excitation temperature of 48⁺¹⁴₋₉ K, which is significantly higher than the apparent dust temperature of T_d = 33 ± 2 K (β = 1.9) derived under the common assumption of an optically thin far-infrared dust emission, but fully consistent with T_d = 52 ± 5 K of a general opacity model where the optical depth (τ) reaches unity at a wavelength of λ₀ = 170 ± 23 μm. Moreover, the general opacity solution returns a factor of ∼2× lower dust mass and, hence, a lower molecular gas mass for a fixed gas-to-dust ratio, than with the optically thin dust model. The derived properties of GN20 thus provide an appealing solution to the puzzling discovery of starbursts appearing colder than main-sequence galaxies above z > 2.5, in addition to a lower dust-to-stellar mass ratio that approaches the physical value predicted for starburst galaxies.