Wanted to buy mobile under 4000 rupees

Since you mentioned seniors, I'm assuming you want a phone for the elderly. You can try these:

And if you can't find any of those, then you can get Android One phones, as suggested by @Kawaljit. If Android is too complex for those who need this phone, then Windows Phone is also a good option. You can go for sub-Rs. 5,000 WP devices such as Celkon Win 400 or Lava Iris Win1.

Smartphones under Rs. 4,000 are generally going to be disappointing, so it is best to hold out for a while, save some cash and get a phone in the Rs. 5,000-6,000 range. The jump in quality is definitely worth the extra cash.