There are a few things I would like to comment on:
1) When my boys were that age (not that long ago), four months was the recommended standard age for starting cereal. Now, I believe the APA has changed that to six months, so there is nothing wrong with waiting a couple months.
2) If you want to save every drop of milk you pump (believe me, I know the feeling) there is nothing wrong with mixing the cereal with water rather than breast milk, and saving the milk for bottle feedings.
3) Do not worry about her getting confused and not wanting to nurse. I have never heard of that happening.
4) I personally do not believe that it will help her sleep better. I know a lot of moms say it does, but I did not find that to be true with my kids. I think it is just coincidence, as babies biologically no longer need to eat during the night starting between 4-6 months. And, the fact that there are fewer calories and less fat in cereal, actually means it is a less satiating food than breast milk, so I do not see how that correlation is possible.
5) And , one last thing (sorry if I am rambling on): Cereal is just a training tool to get a baby used to mouthing and swallowing something thicker than breast milk or formula. It is not a nutritional food and should not replace breast feedings at this age, even if you choose to introduce it now. It is not meant to satiate her hunger, it is meant to train her to eat solid food. If she is still hungry, she will nurse longer and induce your body to produce more milk to meet her new demand.