I think they’re too similar too. I’m a big fan of [name_u]Quinn[/name_u] for a girl’s name, though, and I like it with your other two children’s names.
[name_f]Savannah[/name_f] is to close to [name_f]Siena[/name_f]. I also think you need to stick to a feminine name, rather than unisex for your sibling set, as I think [name_u]Quinn[/name_u] feels masculine with the gorgeously feminine [name_f]Siena[/name_f].
Also you could go with place names as a subtle theme that connects them as there is [name_u]Hudson[/name_u] Michigan (also [name_u]Hudson[/name_u] [name_u]River[/name_u] of course), [name_f]Siena[/name_f] [name_f]Italy[/name_f], and…
[name_f]Charlotte[/name_f] ([name_u]North[/name_u] [name_f]Carolina[/name_f])
[name_f]Geneva[/name_f] (Switzerland)
[name_f]Florence[/name_f] ([name_f]Italy[/name_f])
[name_f]Lucia[/name_f] (St [name_f]Lucia[/name_f], Caribbean)
[name_f]Victoria[/name_f] (Australia)
Alora (Spain)
[name_f]Valencia[/name_f] (Spain)
[name_f]Alexandria[/name_f] ([name_u]Egypt[/name_u])
[name_f]Helena[/name_f] ([name_u]Montana[/name_u])
[name_u]Skye[/name_u] ([name_f]Isle[/name_f] of [name_u]Skye[/name_u], [name_f]Scotland[/name_f])
[name_f]Amelia[/name_f] (Sea Islands [name_f]Florida[/name_f])
[name_f]Cara[/name_f] (Inner Hebrides [name_f]Scotland[/name_f])
[name_u]Carmen[/name_u] ([name_f]Laguna[/name_f] de Términos, Campeche Mexico)
[name_f]Caroline[/name_f] ([name_f]Line[/name_f] Islands Kiribati)
[name_u]Clare[/name_u] (Clew [name_u]Bay[/name_u] Islands, [name_f]Ireland[/name_f])
I think [name_u]Quinn[/name_u] goes with [name_f]Siena[/name_f] and [name_u]Hudson[/name_u] better than [name_f]Savannah[/name_f] but [name_f]Savannah[/name_f] is still cute. I love Savi for a NN. [name_f]Savannah[/name_f] [name_u]Quinn[/name_u] is also cute!