I Penelope is wonderful, and I don’t think she would feel too out of place. Though she most likely will receive a shorter nickname that would match the length of her sisters…
From the suggestions so far my favorites are…
Cleo/Clio
Echo
Iris
Hero
Eris
Leto
For me sound is a bigger factor than length, and all of these names meet your requirements for greek, four letter words but also have different sounding endings then Maia & Zoee
But if that doesn’t bother you so much I also really like…
Hali
Clea
Lyra
Iole
Ione
Some others you might consider that might not entirely meet requirements…
Selena
Xanthe
Irene (prn. eye-ree-nee)
Ianthe
Eos
Delia
Daphne
Briseis (bree-sees)
Ariadne
Aoide (eye-du)
Aella (prn. similar to Isla)