I'm gonna ans your question no. 4 and 5.
Ans4- Are you asking about 4 ppl having waist of 35-40 or 27-30? no one can ans it satisfactory. you're the one who knows your family members the best. go to showroom, sit on rear seat and have the idea of space available for rest 3 ppl. thats it. you can easily judge that can 4 ppl OF YOUR FAMILY sit easily in back side. so go to showroom and check your self.
Ans5- Features of Vitz:
4 power win + steering, ABS, Dual Airbags, Auto trans available, spacious, comfortable, strong, fuel effeicient.
Feature of Chevy:
2power win(front) + steering, Spacious, Comfortable, strong, fuel effecient.
Feature of Santro:
2 power win(front) + steering, fuel efficient.
These are the sailent feature of these 3 cars. If you can afford Vitz then go for it otherwise go for Chevy.