C.C.
I always thought my 4 year old daughter was a light sleeper but she has to share a room with her 7 month old brother who usually wakes up once or twice before morning and he will sometimes cry for a few minutes and she does not even flinch. You may be surprised about how she will react and she may just get used to it after a month or so. I would give it a try and see what happens, if it's too hard on her then you might just have to wait a little longer. If you get to the baby as soon as she wakes up then the other child may not have time to fully wake up and realize whats going on. Good Luck