It IS confusing! A good rule of thumb for breastfed babies is that they take between 18-24oz of breastmilk a day. I'd recommend assuming the higher end of the scale for safety's sake. Divide that into the number of feeds and that's roughly how much to put in his bottles. I'd personally always add an extra ounce just to be on the safe side.
i.e. if baby is having 8 feeds a day, then they're likely taking c. 3oz per feed. If they're having four feeds, then they're likely taking up to 6oz per feed. Does that make sense?
However, having said that, if he's used to taking 4oz, his stomach capacity isn't going to suddenly increase to comfortably take 6oz over night so I'd recommend still making up the larger bottles but just not pushing any more once he starts to lose interest