I think [name_f]Susanna[/name_f] & [name_f]Fern[/name_f] would be stunning with your older girls very southern [name_f]Belle[/name_f] 
[name_m]Other[/name_m] ideas could be [name_f]Juniper[/name_f], [name_f]Magnolia[/name_f], [name_f]Daphne[/name_f] & [name_f]Gemma[/name_f].