There are loads of well known 4 letter names for girls - Lara, Sara, Tara, Cara, Mara, Zara, Anna, Nina, Emma, Ella, Edie, Evie, Alma, Alva, Elsa, Etta, Nell, Gwen, Tess, Gaia, Maia, Kaia, Hera, Cora, Dora, Lily, Orla, Lyra, Lila, Mila, Aria, Rhea ... I could keep going, you are really spoiled for choice.
Boys are much harder (a lot are variants of John or nicknames) - Sean, Evan, Owen, Euan/Ewan, Ioan, Iolo, Luke, Mark, Seth, Jack, Zach, Jago, Milo, Arlo, Theo, Nico, Cole, Ivor, Paul, Saul, Orin.