  We prove that the degree 4 sum-of-squares (SOS) relaxation of the clique
number of the Paley graph on a prime number $p$ of vertices has value at least
$\Omega(p^{1/3})$. This is in contrast to the widely believed conjecture that
the actual clique number of the Paley graph is $O(\mathrm{polylog}(p))$. Our
result may be viewed as a derandomization of that of Deshpande and Montanari
(2015), who showed the same lower bound (up to $\mathrm{polylog}(p)$ terms)
with high probability for the Erd\H{o}s-R\'{e}nyi random graph on $p$ vertices,
whose clique number is with high probability $O(\log(p))$. We also show that
our lower bound is optimal for the Feige-Krauthgamer construction of
pseudomoments, derandomizing an argument of Kelner. Finally, we present
numerical experiments indicating that the value of the degree 4 SOS relaxation
of the Paley graph may scale as $O(p^{1/2 - \epsilon})$ for some $\epsilon >
0$, and give a matrix norm calculation indicating that the pseudocalibration
proof strategy for SOS lower bounds for random graphs will not immediately
transfer to the Paley graph. Taken together, our results suggest that degree 4
SOS may break the "$\sqrt{p}$ barrier" for upper bounds on the clique number of
Paley graphs, but prove that it can at best improve the exponent from $1/2$ to
$1/3$.
