I have same issue. Only this past couple weeks and my baby is also just turning 4mo. I think mummy is there or not, she will still refuse the milk or play with the teats unless she is very hungry. But my baby is overweight and 2 PDs that i went said most probably baby is "adjusting" the weight. So just follow baby's need or prolong the feeding time.
I find that dreamfeed is helpful if you really want your baby to drink. Read more