Formula or vegetable water for 4months.

I have a 4months daughter, this time her weight is not increasing. Her current weight is. 6.03kg and last months was 5.67kg. Her birth weight is. 2.89kg. Is it fine or not. She is taking my breast milk every hr and after 5 to 10min she is start crying some time. May be my breast milk is not sufficient for her and she is still hungry. Can i start formula milk with breast milk. My doctor advised me to start vegetable water with salt and rest spices except red chilli but am confused bcoz as per my knowledge water should be start after 6 months. Does it impact my baby health. Kindly suggest should i start formula milk or vegetable water with salt and all spices (except red chilli) at the age of 4months child.