Hi K.,
Congratulations on your little angel!I agree with the mom before me, babies do a lot of growing while sleeping, and 3 to 4 hr even at a time for nap is normal. Babies are all different, my firts would take naps like her, and like you I would just feel like waking him up to eat. I had a great Ped that advised me that this IS important during the first weeks while they develop the "hunger feeling", then is not necessary, they will wake up when they are hungry. Seems like you are doing great with her. I understand your apprehentions though. I also had twins, born at 5 pound so is only natural ou are worried about waight gain and all. But for what you say seems like she is doing great! As for the naps on her crib, I wouldn't even worry, that will happen eventually, the important things is that she is sleeping. I actually felt better ,with my twins specially, taking their naps close to me. I used a play pen that I put on my living room so I could do my things and be able to check on them everytime I went by. It wasn't long until they went to the cribs as well for naps, maybe around 6 moths or so. And please, DO NOT worry about spoiling her, she will be on her crib for naps before you know it. As it is I think in a blessing and not that common, that she is sleeping wonderfuly through the night on her crib.
Also, not to belittle other's advise,but before adding anything like fish oil or so to her diet, consult with your ped. Fish oil does provide omega 3 that is great for brain development, but I am not sure you can just incorporate that in the diet of an infant,her digestive system is also in development and if you are bfeeding, your milk has by nature, all the nutrients that she needs to grow healthy and strong, and if she is on formula,I am sure you are giving her the right one, as you say that her weight gain is normal...Just my opinion.
I hope this helps you!
Many blessings!