This volume constitutes the refereed proceedings of the 4th International Conference on Optimization and Learning, OLA 2021, held in Catania, Italy, in June 2021. Due to the COVID-19 pandemic the conference was held online. The 27 full papers were carefully reviewed and selected from 62 submissions. The papers presented in the volume are organized in topical sections on synergies between optimization and learning; learning for optimization; machine learning and deep learning; transportation and logistics; optimization; applications of learning and optimization methods.

This volume constitutes the refereed proceedings of the Third International Conference on Optimization and Learning, OLA 2020, held in Cadiz, Spain, in February 2020. The 23 full papers were carefully reviewed and selected from 55 submissions. The papers presented in the volume focus on the future challenges of optimization and learning methods, identifying and exploiting their synergies,and analyzing their applications in different fields, such as health, industry 4.0, games, logistics, etc.

Hawaii Manuela's small Spanish village buzzed with tales of life in a faraway land free from starvation and angst. In the early months of 1911, with nine children and four Silvan Hernandez (and Gonzales) families, they boarded a British immigrant steamer, the SS Orteric, bound for the Hawaiian Islands. Sugar plantation owners wanted immigrants from Portugal and Spain to work their plantations. They paid for passage, guaranteed work for them, school for their children. In a starving and poor time where the military brandished a strong arm, the families took a gamble along with other families in their village; a mass exodus of friends and family---leaving everything they knew---sometimes everyone they loved. Manuela's epic immigration story is filled with tragedy and triumph. Chosen to watch over her brothers as the family makes their way south to La Linea at the Rock of Gibraltar, she was sure her heart would break into pieces. Living through the trials of traveling through Spain to the coast, a place she'd never seen was a nightmare and a dream. An ocean, ships, big cities and fears waited. The quagmire of traveling in steerage for two months added to her grief but the beauty and world of flowers in Hawaii lured her into bits of happiness she hadn't imagined. And meeting her young man in Hawaii and finding him again in California gave her the intensity of life that the trek from Spain promised. This lively memoir is based on the author's grandmother; Spain and Hawaii come alive and encompass five generations, a narrative non-fiction laced with embellishment.