My DD was a bit younger (2yrs 3months) when I had DS but we made a HUGE deal about how much fun she was going to have with her Nonna/Nonno when I went to hospital. We chose a new overnight bag for her and packed a new coloring book/pencils for her and had it ready for when the time came. Also we kept telling her that she could come everyday with Daddy to see me and her new brother etc etc. Also took her shopping and she chose a present for her new brother that we then wrapped and kept reminding her that she could bring it to the hospital and give it to the new baby. Basically we just kept re-inforcing all the positive things and in the end she was busting for me to go to hospital lol!
Good luck!