Grrrrr! Another 'health' professional telling mothers their milk quality is poor.
Scotsbird, Pupuce is right - your milk quality is just fine and as long as the baby is feeding often and effectively he will get milk that is just perfect, including all the calories he needs (not heard this about breastmilk being higher in calories, than formula, though, Pupuce - my info is that on average, they're about the same. But breastmilk is certainly easily digested).
A four ounce top up for a baby under 7 pounds is huge and doing that a few times a day would kiss goodbye to breastfeeding....did your HV not think of that??
A baby who's on the small side is likely to need frequent feeding in order to grow well at first. Lots of feeds is good, and sleeping for 5 hours is not - that means there was probablt a gap of six hours between feeds, thus missing out two breastfeeds and reducing the stim. of the breastmilk 'production line' ......result: less breastmilk.
But honestly, it sounds as though your HV neither knows nor cares about this
She may be a lovely person, and kind, and committed. But she doesn't know much about bf.