I really love your current names - all very pretty individually and go together well.
I don't think you should go overly nature/bohemian/vintage because the 5th name will stand out as not going with the others.
How about Hannah, Naomi, Phoebe, Florence and:
Astrid, Alexia, Amber, April, Bonnie, Carys, Cara, Cassandra, Clarissa, Daphne, Delilah, Erin, Erica, Eliza, Georgina, Grace, Giselle, India, Imogen, Ivy, Jonquil, Jemima, Joanna, Juliet, Kate, Livia, Larissa, Miranda, Martha, Rowena, Romilly, Ramona, Rhiannon, Sorrel, Saskia, Serena, Tabitha, Tamsin, Taryn, Violet, Verity, Xanthe, Zara.
Aidan, Arlo, Barnaby, Cosmo, Christian, Elliot, Ivan, Jeremy, Josiah, Jonty, Kenneth, Lewis, Milo, Micah, Merl, Owen, Quentin, Robin, Rowan, Sebastian, Seth, Titus, Vincent, Victor, Wilfred, Xavier, Zachary