POLY-CHAR 2019, POLY-CHAR 2021, POLY-CHAR 2022 and POLY-CHAR 2023 were held in Nepal [Kathmandu], Italy [Venice], Germany [Siegen/Halle], and New Zealand [Auckland], respectively. POLY- CHAR 2020 could not be organized due to the effect of the COVID-19 pandemic. The reports of the conferences are included.