Add a stop is the only way to do this, there is no different "via" option.
via = add a stop = add a via point
- Select your final destination first (City B),
- Start navigation,
- Search for your desired "via point" (City C), select it,
- On the pop-up click "Add a stop" instead of new destination, it will become your "via"
That's the only way you can do this. Regarding asking Waze for a fourth route, three is the current maximum.