S.R.
Hello S.. With my 4 y/o I let her be involved with the pregnancy. She went to the Dr with me (primarily b/c of lack of babysitter, but it was so great) and the Dr would listen to the baby's heart beat out loud then he would put it on her heart (with her consent) and she could hear her heart beating. She went to the sonograms, helped with the shoping and got to choose things. She became the big sister before the baby was even born. I was worried about her b/c she had been the youngest child and only grand-daughter with my grandparents. I hope that helps with your current pregnancy.
My baby is now 7 months old and my 4 y/o does get upset when the baby takes something that is hers. I have been enforcing (since the end of my pregnancy) that she has to keep her toys in her room so the baby doesn't get them (I was worried about the small toys that the baby would choke on) and when she brings toys in the livingroom she knows the baby might get the toy and play with it. Also, she likes playing the baby's toys. I tell her that she will have to share her toys if she is going to play with the baby's toys.
IMO, your 4 y/o is feeling shafted with the new babies. She might not have enough alone time with mom and dad. I would try the time alone technique. And I greatly recomend involving her in the new baby's pregnancy. You could make that your alone time and grab a lunch at McDonalds with a play gym or something like that. And let her pick out some clothes or toys so she feels important in the baby's life from the beginning. Best of Luck!