Breastfed newborns NEED to eat very very frequently. Nursing virtually non-stop for the first few days is expected, often hour long nursing sessions every hour or so. that is because their stomach starts out at the size of a marble, about 5 ml, or one swallow of amniotic fluid.
Then their stomach stretches to the size of a shooter marble about the time your breastmilk begins to come in. then they will nurse for a bit shorter and not quite as often, but they aren't really good at it, so they may nurse for 45+ minutes every hour or 2.
Stomach size continues to stretch as your milk comes in, the size of baby's fist or up to about 2.5-3 oz. at that point you may see them eating every 2.5-3 hours, but they may still take 30-40 minutes to eat that much.
now if you are feeding a 3? week old baby 4 oz at a time, that is TOO MUCH. see a breastfed baby on average takes i only 25 oz of breastmilk at the breast per day. and that is regardless of how old they are or how big they are, although when they are young, they may take in on the low end of the range of 19-30 oz a day. So at 4 oz at a time, your baby should be eating only about 6 times a day, but a newborn needs to eat 10-12 times a day.
A pump is not efficient, how much you get by pump is not how much baby gets at the breast. but also a pump doesn't empty a breast the same way a baby does--a baby uses manual manipulation plus suction to empty the breast, the pump only uses suction to empty the breast. Also that 10-12 times a day is what it takes to establish your milk supply.
If you are set on pumping and feeding breastmilk by bottle, then you need to do a few thing to be protective of your milk supply. rent a high quality hospital grade double electric pump, pump often enough, at first every 2 hours for at least 24 hours straight, then 2-3 hours during the day and one 3-4 hour sleep at night, or at least pump as often as baby eats. pump long enough, if you don't seem to have enough milk (25 oz a day is your goal remember) then you eed to pump 5-20 minutes after the milk stops flowing to signal your body you need a higher milk supply.
any milk remaining in your breasts for long signals your body to make less milk. very frequent and often long nursing sessions with a newborn is what sets the stage for a good milk supply--this is when your boobs are growing milk ducts and alveoli for producing future milk for baby. sitting down and nursing baby sa often as possible and as long as they need to nurse in the beginning is laying the ground work for future success.