I grew up in an all girl generation, only sisters and female cousins - congrats on your gaggle of girls! <3
I honestly like all of them and it's very close, but by a hair I'd vote Sienna because it's the most different.
some other ideas you may like:
Zara, Olivia, Rosie and Celeste
Zara, Olivia, Rosie and Bailey
Zara, Olivia, Rosie and Abigail (I actually love Abigail Ivy)
Zara, Olivia, Rosie and Piper
Zara, Olivia, Rosie and Eden
Zara, Olivia, Rosie and Brooke
Zara, Olivia, Rosie and Tess(a)
Zara, Olivia, Rosie and Juliet
Zara, Olivia, Rosie and Thea nn Teddy if she likes
Zara, Olivia, Rosie and Wendy
Zara, Olivia, Rosie and Belle
Zara, Olivia, Rosie and Caroline
Zara, Olivia, Rosie and Felicity
Zara, Olivia, Rosie and Annika nn Annie/Nik
Zara, Olivia, Rosie and Grace
Zara, Olivia, Rosie and Mary-Jane
Zara, Olivia, Rosie and Davina
Zara, Olivia, Rosie and Meg (alright 4 girls I can't help but go Little Women lol)
Zara, Olivia, Rosie and Blair