Can I use Apple Watch S4 (GPS + Cellular ) bought in Japan in India? (Not about the Warranty)

In theory yes as both countries use the same watch model. However it will be entirely up to an Indian carrier to activate it on their cellular service.

If the Indian carriers won't activate it, it can still be used as a GPS only watch without activating the cellular portion.

This might answer your question.....https://www.quora.com/Will-the-Apple-Watch-Series-4-cellular-work-in-India-In-which-country-should-I-buy-it