Bro,
$R_3 \leftarrow 7*R_3-5*R_2$ will change value of determinant by a factor of $7$.
Similar,
$R_4 \leftarrow 7*R_4+2*R_3$ will also change value of determinant by a factor of $7$.
You're getting this new determinant $=49*4$.
So, the original determinant should be $=\frac{49*4}{7*7} = 4$.