Our situation was different than yours because our son was younger. However, even with a 4 year old I probably wouldn't have said anything this early in the process. This can take a really long time and they just don't understand that. If it were me, I'd probably just keep saying it's going to be a really long time IF she brings it up. I also wouldn't talk about it around her. However, that's me. I try and limit how early I tell my little kids about stuff, they just get too worked up about it. At that age we probably only gave our son a week's notice before vacations. We'd pull out a calendar and draw a picture of the first travel day then mark off each day before that.
In case it helps someone, here is what we did:
Our son was 2 when we started the adoption process. For a lot of reasons we were going to limit the search until his 3rd birthday. If no baby by then, we would stop looking. Because he was so young and because we were not assured that we would adopt, we did not tell him about our intentions. We did read new baby books intermittently (just as we might rotate between truck books or alphabet books) and I held a baby as often as I could get my hands on one, but that was all the prep we were able to do.
We ended up getting a baby when our son was 31.5 months. We got a phone call one morning about a baby that was less than 1.5 hours old. We met her at 6 hours of age. We left our son at home with a sitter and told him that we had to go help someone. When my husband went back to bring him to the hospital, he said I was helping a baby.
The birthmom couldn't sign until 72 hours then we had to wait until the unknown birth dad was TPR'ed at day 70. We did not want to bring home a "sister" only to have to give her back, so we kept telling him that we were taking care of her. (On the off-hand chance we had to give her back, we didn't want him to think that because his "sister" had to leave he might have to leave as well.)
When she was finally legally free, we were only a month from finalization. I think we waited until a week before finalization to tell him what was happening.
There is a big difference between 2.75 and 4.