Somos-4 and Somos-5 are arithmetic divisibility sequences

We provide an elementary proof to a conjecture by Robinson that multiples of (powers of) primes in the Somos-4 sequence are equally spaced. We also show, almost as a corollary, for the generalised Somos-4 sequence defined by ${\tau }_{n+2}{\tau }_{n-2}=\alpha {\tau }_{n+1}{\tau }_{n-1}+\beta {\tau }_n^2$ and initial values ${\tau }_1={\tau }_2={\tau }_3={\tau }_4=1$, that the polynomial ${\tau }_n(\alpha ,\beta )$ is a divisor of ${\tau }_{n+k(2n-5)}(\alpha ,\beta )$ for all $n,k\in \mathbb{Z}$ and establish a similar result for the generalized Somos-5 sequence.