We present new observations, carried out with IRAM NOEMA, of the atomic neutral carbon transitions [CI](1-0) at 492 GHz and [CI](2-1) at 809 GHz of GN20, a well-studied star-bursting galaxy at $z=4.05$. The high luminosity line ratio [CI](2-1)/[CI](1-0) implies an excitation temperature of $48^{+14}_{-9}$ K, which is significantly higher than the apparent dust temperature of $T_{rm d}=33pm2$ K ($beta=1.9$) derived under the common assumption of an optically thin far-infrared dust emission, but fully consistent with $T_{rm d}=52pm5$ K of a general opacity model where the optical depth ($tau$) reaches unity at a wavelength of $lambda_0=170pm23$ $mu$m. Moreover, the general opacity solution returns a factor of $sim 2times$ 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.
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