smc

Denote the sum of the first $2$ terms as $x$. Since we know that the sum of the first $6$ terms is $91$ which is $7\cdot 13$, we have $x$ + $xy$ + $x{y}^2$ = $13x$ because it is a geometric series. We can quickly see that $y$ = $3$, and therefore, the sum of the first $4$ terms is $4x=4\cdot 7=\boxed{28}$