I want a petrol manual car with a 5-star GNCAP safety rating, at least four (or another even number of) cylinders, excellent fuel efficiency, and long-term reliability so I can run it for around 20 years. I’m new to earning and have a limited budget. Please suggest suitable options.

Looking at your requirements of reliability and fuel efficiency, along with a good safety rating, we would recommend you look at the Maruti Suzuki Dzire. 

It is a three-cylinder engine; however, this should not be something you should worry about as the unit has adequate performance, in fact, a very good low-speed response. 

The Dzire is a sedan, but it fits within the sub-4 meter footprint, and that, along with its light controls, will make it an easy drive for you.