As far as I know, Polyvisol is just Vitamin supplement. I let my 12-mth-old take it everyday for 增強抵抗力 & prevent from flu.
Baby could resists milk because of teething and is always a headache. Given your baby is already 4-mth-old, you can let him try 米糊/粥仔 or try even feed him/her some soft bread fully-soaked with milk. Hope he/she will like it!