Don't worry, you didn't ruin your child by feeding her cereal at 4 mos. A lot of doctors are saying 6 mos now I believe, but 2 years ago my doctor gave me the go-ahead at 4 mos too. I ended up feeding mine cereal just before 5 mos old. Since then I have read a lot about their stomach and intestines not being fully formed, so I plan to ask my doc next time before I feed at 4 mos. But mine was starving one day all of a sudden and did very well on solids. When mine was that little, the whole point of waiting was because they thought introducing food too early caused allergies, but they have since retracted that advice as incorrect. Things change constantly, and there are very few "rules".
You've gotten good advice about how she is learning to eat and not actually rejecting it. I have found all that to be true. One sign of readiness for eating is whether or not they can actually refuse to eat, and at six mos she should be doing that if she really doesn't like it. What McMama said was exactly what I experienced with my son.
Since she has been on the cereal for a while, I would go ahead and start mixing it with something else. I did cereal for several weeks and then did veggies. Did your ped give you any guidelines on what to try next? Mine suggested cereal, then yellow veggies, then green veggies, then fruits. There are a few things that are better to wait on based on the acidity and easiness to digest, like tomatoes, strawberries and yogurt. I'll post my favorite baby food websites at the end and they have great info about that kind of stuff. Starting with yellow veggies worked very well for us. After veggies and fruits, my son didn't like cereal by itself much anymore.
Make sure you follow the 4-day wait rule for allergies and other reactions. I did one meal a day for a month or so, then upped it to two. With 2 meals a day, I would use one meal to feed a couple ounces of something he'd already been introduced to, and the other meal to introduce something new. I just followed his cues as to when he seemed to need more feedings per day. Make sure you are offering breastmilk or formula before each solid food feeding, that is where she should get her real nutrition. Then about 20 mins after "milk" do the solid. If you are nursing this is especially important to not diminish your milk supply.
Even if you don't want to make your own babyfood (which is waaay easier than it sounds) the book Super Baby Food by Ruth Yaron has great guidelines for when to feed things and how much. I didn't follow all her advice for making food, I did watered down version and didnt make my own yogurt or anything. But it was a great resource and very helpful for figuring out what to do next and how to prepare food. The websites I like are homemade baby food websites too, but like I said, they have good info on ages to introduce things and what tastes yummy mixed with what.
I hope all that helped, and I am sure you are doing fine. Don't stress out over feeding (easier said than done, but now I wonder what I was so worried about, looking back it was pretty easy :) Just enjoy this fun new experience, razzberries and all!
http://www.homemade-baby-food-recipes.com/index.html
http://www.wholesomebabyfood.com/
www.weelicious.com