My youngest is worried for the new baby?

I have three daughters, five year old twins and a 3.5 year old. The twins are used to taking care of each other and their little sister and are excited to tell their friends about the baby (due really any day now!) but Coco, my youngest, is worried - of course, she won't be the 'baby' anymore and with two older sisters and the rest of our family always doting on her, she seems to really be bummed about losing that spot. We've psyched her up - along with our older two, as to not leave anyone out - and allowed her to help choose names, clothes, colors, toys, etc. and I encourage her and talk very enthusiastically about 'Coco being mommy's helper when Baby Sister comes!' but she only gives a little smile. She isn't outspoken whatsoever, and is very very shy, so she doesn't have fits or resort to childish behavior - I can just sense her woes. She's a good girl and won't get in the way and try to steal attention, but i think that makes it even harder because she may suffer in silence. Any unique ideas?