Forever In Blue : The fourth summer of the sisterhood (book 4) by Ann Brashares

With unraveled embroidery and fraying hems, the Traveling Pants are back for one last, glorious summer.
Lena: Immerses herself in her painting and an intoxicating summer fling, fearing that the moment she forgets about Kostos will be the moment she sees him again.
Carmen: Falls under the spell of a sophisticated college friend for whom a theatrical role means everything and the heritage of the Pants means nothing.
Bridget: Joins a dig for an ancient city on the coast of Turkey and discovers that her archaeology professor is available in every way except one.
Tibby: Leaves behind someone she loves, wrongly believing he will stay where she has left him.