Answer
$s_1 = 3.6$
$s_2 = 8$
Work Step by Step
Recall:
An angle of $\theta$ radians subtends an arc of length $\theta r$ on a circle of radius $r$
Let $s_1$ denote the length of the arc subtended by $\theta$
$$s_1 = r\theta \\[3mm] s_1 = 4 \times 0.9 \\[3mm] s_1 = 3.6$$
Let $s_2$ denote the length of the arc subtended by $\phi$
$$s_2 = r \phi \\[3mm] s_2 = 4 \times 2 \\[3mm] s_2 = 8$$
