Its quite common for babies to sick up a little after feeds - or even to throw up a whole feed that they've happily eaten. Some babies are sick a bit after almost every feed, others don't.
At the newborn stage they're still learning themselves how to feed, etc. And its probably more likely that they've swallowed some air and then puke up the feed that's on top of that rather than they're just overfeeding.
At four weeks I would relax, go with the flow. Don't worry too much about how much your baby is feeding or how often. You can offer your finger - they will suck on that. If the baby starts to cry then you'll know that actually they are hungry. If they happily suck on the finger then they're not hungry and just want to suck.
With breast fed babies they do go through very definite growth spurts where they do want to feed all the time and are constantly sucking on you - this is normal, but you can end up thinking that they can't possibly be hungry.
My daughter had a lot of wind from breast feeding and in fact she didn't want to feed at all when that happened. And you could tell that she was uncomfortable rather than hungry because she would pull her legs up. And after about half an hour or an hour or so she'd calm down again. Whereas if she was really hungry I don't think she'd have calmed down again.
Not sure this makes too much sense - basically don't worry about overfeeding your baby at this stage in their life.