The laptop actually supports upto 16GB, however Apple sold it with a max of 8GB, and that's what they supported. I myself have it upgraded to 16GB since 2013, and it works fine. Crucial’s site documents it too https://www.crucial.in/compatible-upgrad...)
I have the core i5 model, and have upgraded to 16GB since 2013, and it has been nice ever since. Take a look at this, https://www.crucial.in/compatible-upgrad...) which tells exactly what will fit in your mac. Unbuffered is fine, and most entries in Amazon India will do just fine. eg. https://www.amazon.in/dp/B00EHJ5U0K and https://www.amazon.in/dp/B00KEAEX54
You can add a 8GB along with your existing 4GB, taking the total to 12GB. You can also add just one 4GB, taking the total to 8GB.