Going for Yunohost on your Pi4 can make things easier, just follow the Yunohost documentation, and later you can ask help in the Yunohost forum if needed : https://yunohost.org/en/install/hardware:rpi34 Instead of Mastodon you can install sometimes more light weight and simple, like GoToSocial : https://apps.yunohost.org/catalog?category=social_media