Between CNG and petrol, CNG is much better in terms of fuel economy. Since you plan to use the car on the highway with four passengers, we recommend an SUV over a hatchback or compact sedan.
Consider the Tata Nexon Smart i-CNG. It is the base model, but as you mentioned, that is not an issue. Its turbocharged engine provides added power on the highway, and its robust suspension ensures comfortable travel for four passengers.