How to measure how much baby is eating?

You don't! People who talk about oz are feeding from a bottle. If your baby is having wet and dirty nappies and is gaining weight then they're eating enough

You can weigh them before and after a feed but it's really not necessary. Babies will take what they need directly from the source. Our milk composition changes over time too, so they won't necessarily require higher volume because the milk will nourish them longer.

My baby drink 3-4 oz

I nurse then the next feed a give a bottle of my milk because I want to make sure he is drinking enough

I just keep note of how long he nurses on each side, but aside from a weighted feed, you have no way of knowing

@Rachel timing doesn't mean much. Not only are every baby and every boob different, they can get more efficient as time goes on. Suggesting to take note of how long might trigger someone to believe that their older babe might not be getting enough if their feeding has become more efficient.