Hi T., My name is D. and I'm a Grandmother now but I do have experience in this area. Children, my oldest, seemed to do best by me having her talk to my pregnant belly and communicating a lot about her being a big sister and what all that means. I also took her to the library and we found books on being a big sister, as well as ones talking in general about another sibling.
We also picked out the outfit together which I was bringing home her sister in.
I have pictures still today of my oldest holding her little sister with such pride.
Four year olds respond well to being a helper at this age as long as it is not over-done. The feeling of being included is important, and makes the older sibling feel a part of. I have seen many families in the past put too much pressure on the older child, expecting them to be responsible, to some degree, for looking out for the younger child interests too much,this is not wise. Especially if they, the older child, gets repremanded for what happens to the younger child. I can speak from experience when I tell you this does not turn out well. There is a great deal of resentment that starts to form in the eldest.
However creating positive and safe environments in which to learn about the new addition is important. Allowing your daughter to play her part in the upcoming siblings life is just important.
Other suggestions, and these are only suggestions, is to take your daughter to social play places (if you'r not doing this already) and show her examples of big and little sisters and brothers, have her interacting with other children in this way is a solid example amongst her peers.
My daughter, who I love dearly and I believe her to be a outstanding mother, has done all of the above examples, she also is good at understanding each childs needs. How each child needs their alone time with a parent to read a book, play a game, brush there teeth, learn to ride a bike or to just sit, snuggle and watch tv. These are healthy ways of working through the transition period.
My daughter is my youngest, she is now 22 and is going on her third child (due the end of March, first of April), she has a degree in teaching but is choosing to stay home and raise her children first.
I commend you are reaching out to other mothers, I believe mothers don't get the credit they deserve. I can tell you are a caring mother. Good luck!