ja99, first, congratulations on your baby!
From what you say it appears that your baby is only feeding every 3 hours; is that right?
A "good 4 hour nap" actually spells trouble in a newborn who isn't gaining weight.
Despite what you may have been told, this is very probably not often enough for your tiny baby. It is very usual for such young babies to feed far more frequently than this, every 1 to 2 hours is completely normal. If your baby was gaining fast, then 3 hourly feeds may present no problem, but when weight is static, you would certainly increase milk intake with more frequent (rather than longer) feeds.
Sometimes babies with difficulties gaining conserve energy by sleeping rather than eating - this is something you can turn around by increasing the frequency of feeds - even more than you have already.
How do you feel her latch is? Do you have any problems with soreness? If so, you should get your latch watched by a trained breastfeeding counsellor (not a HV or midwife) as this will affect milk transfer into your dd.
Do you offer both sides at each feed? When she stops actively sucking on one side, offer the other as the increased flow may wake her up and ensure more sucking. You can repeat this swapping as many times as you want - it's called switch nursing.
Also have you tried breast compressions? I'll find a link in a minute.
I'd advise stripping off naked to the waist and having a skin to skin babymoon, where she has instant access to your breasts round the clock too. Do you have a sling to carry her in?
She hasn't lost weight - which is a good thing. There are lots of things you can do to increase her milk uptake.