Car A drives east at 20 m/s, and car B drives north at 15 m/s…

Step 1. Relative velocity. $\vec{v}_{BA} = \vec{v}_B - \vec{v}_A = (-20\hat{i} + 15\hat{j})\ \text{m/s}$: $20\ \text{m/s}$ west and $15\ \text{m/s}$ north.

Step 2. Its size and direction. $\sqrt{(20\ \text{m/s})^2 + (15\ \text{m/s})^2} = 25\ \text{m/s}$, at $\tan^{-1}\dfrac{15\ \text{m/s}}{20\ \text{m/s}} = 36.9^\circ$ north of west.

Step 3. Separation. Seen from car A, car B moves straight away at $25\ \text{m/s}$. After $4.0\ \text{s}$ they are $25\ \text{m/s} \times 4.0\ \text{s} = 100\ \text{m}$ apart.