what gift would be appropriate

First of all I would like to extend my good wishes to your brother and your bhabhi in advance! Sandeep, you should have mentioned your budget as well so that it was easier for me to guide you. Traditionally, for the fourth marriage anniversary flowers and silk are always a good way to go. Silk comes in a wide variety of applications; ideas include silk clothing, even bed sheets.If you wanna go for modern way be creative and think of modern appliances which you can afford. For your Bhabhi's B'day you can also give her flowers and alongwith that you can gift clothes or kitchen equipments nice peice of ear-rings. hope my suggestion helps you out.

For a wedding or anniversary, you can buy an album; a vase or a photo frame for the couples. For your sister-in-law, you should know her at least a bit after she married your brother. She can buy her a handbag or something you can use in the kitchen. Hope this help.