Hi. Your location definitely changes my answer in that I’m not quite sure about the market or marketablilty of a 4-year degree in something like Exercise Science versus a Certification only, or a Certification and 2-year Associates. If you are located in India and are planning to practice there as well, I think that you are better served by researching the local requirements in India and also by checking with a few Certified and experienced Personal Trainers in the locale that you are interested in working in.
The viability of your fitness education and the need to obtain a certain level of classroom or book education in the field is very locale specific, so before you take the time, effort and expense of taking either track that you mention in your question, if I were you, I’d check what the local trainers there in India do and what the general requirements are there.
I hope that this helps, and good luck!