  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 $T_{\rm
d}=33\pm2$ K ($\beta=1.9$) derived under the common assumption of an optically
thin far-infrared dust emission, but fully consistent with $T_{\rm d}=52\pm5$ K
of a general opacity model where the optical depth ($\tau$) reaches unity at a
wavelength of $\lambda_0=170\pm23$ $\mu$m. Moreover, the general opacity
solution returns a factor of $\sim 2\times$ 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.
