Hi there,
it is not possible to set this pricing directly in Car management or Special routes. Special routes are meant for the whole car rental regardless of the number of people in the vehicle. You create Special routes from POI to POI eg. from a single airport to a single hotel. Here is more information about Special routes:
https://kanev.com/blog/21-how-to-use-taxi-booking-for-fixed-prices-between-destinations
However, you could create a custom field named eg. "Additional passengers over 4" - Option list type - Display on: First booking step - Show on Drop off: Yes - POI Category (select all by holding the Ctrl button) - Field options: New and create all additional seats with a number and price eg. Title: 1 - Price: 5; Title: 2 - Price: 10; Title: 3 - Price: 15 and so on.
You can set this field as Mandatory or not but if it's mandatory it is a good idea to include option Title: 0 - Price: 0 for cases that there are no additional passengers.
Hope this helps.
Kind regards.