Ok, I'm no expert & only telling you what I've heard... but Your baby sounds pretty perfect.
The best signs are wet nappies and the poop changing from meconium to yellow-curdy. You want to see yellow poop by the end of the first week. You want to see a return to birthweight or better, by the time the baby is 2 weeks old.
At 4 days old the milk is still really thick.. partly colostrum, still. And very fatty (I know because I've expressed it off at that point...). As for not always sucking aggressively, that's because the baby is waiting for a little more milk to be made and then she can have it.
When breasts seem to fill up after a sleep or in the hours since a feed, that's often the most watery milk. But when the breasts feel empty and baby just hangs about, that's because the milk the breasts make when they feel almost empty is actually the most fatty milk. In other words, by not aggressively sucking, the baby is just waiting around for a little more fatty milk to be made.
Do everything you can to make your life easier at this point, and get as much rest as you can. Learning to feed lying down is a real lifesaver, for instance.