  We prove that for any $\epsilon>0$ and any trigonometric polynomial $f$ with
frequencies in the set $\{n^3: N \leq n\leq N+N^{2/3-\epsilon}\}$, one has $$
\|f\|_4 \ll \epsilon^{-1/4}\|f\|_2 $$ with implied constant being absolute. We
also show that the set $\{n^3: N\leq n\leq N+(0.5N)^{1/2}\}$ is a Sidon set.
