I would definitely switch to a better brand with no corn. There's several discussions on food and brands if you look back on the discussions. It may cost a slight bit more but they will be eating less. She only 4 months so she still needs puppy food though. Don't let the cute pics fool you...you need to read the labels. This will not automatically mean she will like it but I know several feed a certain brand here but occasionally switch flavors. I usually feed salmon but once every 4 bags or so I may use chicken as my dogs seem to like the switch also! Good luck and don't forget to switch slowly over a week or two!
She may be having trouble with teething. I think the FAQ has links to food ratings if you are worried about quality. You may want to look in her mouth and see if it looks sore. If so, I would give her cold toys to chew on and maybe soften her food. Also, puppies go through growth spurts and will sometimes not be as hungry as others. That is why I like giving them 15 or 20 minutes to eat and then pick the food up. I used to freeze knotted rags for mine(just make sure she doesn't eat them) and give them smaller ice cubes. As long as she is growing and the vet thinks she looks healthy, I would not worry too much about an occasional light meal.
Yes, teething puppies often go through stages of not wanting to eat, and the fastest way to create a picky eater is to add treats to their food. But I also agree with Jane that you need to find a good, high quality food. I've found the easiest thing to do is go to a feed and seed type store and tell them you a looking for a really high quality dog food. It will be more expensive per bag, but you will feed slightly less AND the money it will save you in vets bills and the years it will add onto your pups life will be priceless.
Whole Dog Journal/ Feb. issue will have the food ratings in it!!!! YEAH! I get my food from a small town hardware store and it's cheaper than going to Pet Expo for me. I went to the dog food name and then did a location/dealers search and found one 20 miles from my home instead of 60!!!