When to give mashed foods

So my four month old baby is not drinking his formula like he always does he usually drinks 5 ounces but now lately he has been drinking 3 ounces so I was thinking maybe it’s because he’s getting tired of milk and wanting solids if that is the case I have a question when would you know they had enough of the solids and that they are full and how would be the schedule with the solid like would you give solid food during the day and then once they go to sleep that’s when you just give them milk how does that work