A.G.
My son is also 4 months old and a couple weeks ago, it seemed impossible to fill him up. He would stop eating at about 5 oz of formula (not much for him) but would still root on everything. So I started him on rice cereal before his doctor recommended it. (3 oz of formula to 3 Tbsp. cereal - pretty thick.) He seemed satisfied, but when I took him to the doctor she pointed out that most of his nutrition needs to come from formula. After some experimenting, I've found that 6 oz of formula with 3 Tbsp. of cereal works best for him; he sleeps well, plays happily, and finishes the bottle easily without wanting more afterward. I also asked the doctor about feeding him poi, and she said it is a wonderful, HYPO-allergenic food (allergies are a common problem with feeding ceereal too early), but that it has no iron, so it can be used to fill him when he's still hungry, but iron-fortified formula should still be the main ingredient of his diet. Hope this helps!
A.