Same here, my 4 week old is taking 6oz too. Although sometimes she does leave an ounce or so. She was 7.15 oz at birth and at three and half weeks weighed 10.9oz. She has chubbed right out! My first dd was exactly the same, was a chubby baby following the 98th centile, taking 6oz at 4 weeks too, but now at nearly two years old she is regular size and has been for some time.
I was told that you cant really overfeed with milk as what they dont need they will bring back up - both my dd's have barely threw up, they dont waste a drop! x