Welcome to YT and Congrats!
Don't worry about her spay, she should be fine and it is really best for her.
I don't have experience for you as I am baby-less (skin kind that is). My 2 fur-babies are 4 y/o as well and very spoiled too
I have heard that you should let her smell the babies things that you have or will have at home right now, and let her rub her scent on them. Then, I have heard to bring a baby blanket (with your new baby smell) home from the hospital before your bring the baby home and let her smell it and rub her scent on it - that's all I have heard so far!
Just take your time...if she is good with 'other' kids, she should be fine
