My little one is 7weeks tomorrow. Honestly I don't really fuss too much about the quantity of milk she consumes. I set a standard of of 3.5oz and if she wants more I give more, if she doesn't drink it all then I don't worry cuz she is the one who rejects it. Do PACE BOTTLE FEEDING so it reduces the risk of overfeeding!!! Just listen to the cues of your baby and don't overstress about the number drank. If baby is gaining weight then you're doing just fine! Sometimes mine can drink 6oz but sometimes she only drinks 2oz. Remember not all feeds are nutritional for them so it can vary very much and thats okay.
My son was having 5oz at 6 weeks. He's now having 6oz at nearly 9 weeks. Try it! You can always reduce down to 4oz again if needs be.
My little one was the same around six weeks. Most the time he finish the bottle sometimes he wouldn’t. It was the same bottle throughout the day so when I noticed the pattern, I would up the ones that he always finished. He’s eight weeks old now, most the time he finishes the 5 ounce bottles. Sometimes he still doesn’t so not all bottles have been upped.