Soyo is a small town and the centre is walkable. Most of the guesthouses, restaurants, and the market are within a manageable distance of each other on foot.
Beyond the town centre, you will need transport. There are shared taxis (candongueiros) that run along the main roads, but routes are limited and a private hire vehicle gives you much more flexibility — essential if you want to reach the estuary viewpoints or the beaches north of town. Negotiate the price before getting in.
There are no car hire companies as such. Drivers with 4WD vehicles can sometimes be arranged through guesthouses, though this is informal. Ask at your accommodation. Motorbike taxis (kupapatas) operate around town for short trips.
The roads within and immediately around Soyo are generally passable, but venture further and a 4WD becomes necessary, particularly in or after the rains.