Since there is no fourth episode, your options are limited but definitive:
When discussing anime that push the boundaries of conventional romance and dive headfirst into the taboo, few titles spark as much heated debate as Aki Sora . Based on the manga by Masahiro Itosugi, the OVA series concluded with its third episode in 2010. However, a persistent rumor and search trend often confuses new fans: aki sora episode 4
The Aki Sora anime was released strictly as rather than a standard television series. Fans often search for an "Episode 4," but the production concluded after three standalone releases: Since there is no fourth episode, your options