Four (plus!) months is a long time for a 3-year-old - and that's how long it will be until she is forced to deal with a baby sister. So give her some time to come to it on her own. When my oldest was 3, and nearing his 4th birthday, he announced that he did not want to turn 4, and so he just wasn't going to do it. We said, OK. That's fine. Do you still want a party? Initially, the answer was no party, no presents, I'm not turning 4. Then, he thought a party would be OK, but he still wasn't turning 4. Finally, about a week before his birthday, he said that he thought he might turn 4 after all - and it was all fine from there. Sometimes, with big changes, they just need some time. (We had moved across the country when my son was three, and he got a baby brother in that year, too. He just needed to feel like he was in control of something, and he chose to try to control his age.) Get some books about new babies (We liked "Baby on the Way" by Sears clan) and put them on her shelf, but don't force her to read them - wait for her to choose them herself. Offer to include her in all the baby prep you do (paint color for the baby's room, assembling furniture, etc.) Ask her if she'd like to come along to your drs appts to hear the baby's heartbeat on the doppler. But allow her to say "no" to all of these offers - and continue to invite her to be a part of things, even if she said "no" last time you asked. (You continue to do all the things to get ready that you need to do, and you don't need to hide these things from her, or drop what you're doing to pay attention to her - if she wants to join you, you'd love that, but if she wants to go play with her blocks instead, that's fine, too, but you need to finish what you're doing. You can make space for her to resist the change without enabling her.)
You can also do some things for her to help assure her of her unique place in your family - does she want a new paint color in her room? When I was pregnant with my youngest, my oldest and I started going to the St. Paul Central library for their saturday morning programs (puppet shows, magic shows, nature presentations, music, etc) - and then once my youngest was born, I left the baby with my husband while my oldest and I continued to go to the Sat morning program - it became our weekly date, just the two of us.
Most hospitals also have a "sibling class" for families who will be giving birth in their facility. The one we did through HealthEast for our son was pretty good.
Let her take her time. If you continue gentle offers of involvement and information, but allow her space to process it at her own speed, she will come around.