f your baby is clearly hungry and you don't have breastmilk, then I would begin supplementing right now. As always, I will get flamed for this advice on here but the baby is hungry now, feed him or her now and then move merrily to exclusive breastfeeding
Thing is with new mums, it can be difficult to tell whether a baby is hungry, cluster feeding, growth spurt, or genuinely has a problem. It can also be extremely difficult to tell if you are producing any milk, as being able to express isn't a reliable measure. Constant feeding is normal, and will trigger more milk and help milk come in, but often mistaken as hunger or lack of supply. It also isn't as simple as formula now, and "merrily" pick up exclusive feeding later. Without the early stimulation it can be easy to get into a negative cycle.
O/p, i agree with pp that lack of poo is of concern. Unless she's still passed meconium recently. Rather than mess about trying fixes or internet advice, get medical advice now. They will weigh, but she's 4 days and loss is normal, it is also very important that someone looks her over and checks for signs of dehydration and other issues.
Gp, hv, walk in,m/w, whatever. She's 4 days old and better to get her seen if you're at all worried.