My BB only 2.08kg at birth, she drink S26 as well, she is now 6.5 months old and 6.4kg now. However, she is a little bit short (around 60mm), and only catch small size when compare to the normal growth curve.
I do have your experience that she drank only 3-4 oz milk at around 4 months time, but nevertheless she drink 6-7 oz milk with 糊仔 now. So you don't have to worry, just keep on.