From The Apple Wiki

Initial release21 February 2020 (2020-02-21)
Final release
RC.2.1 / 15 July 2020 (2020-07-15); 3 years ago
Operating systemiOS
Available inEnglish

rootlessJB4 was a semi-untethered jailbreak for iOS 12.0-12.4.8. Notably, unlike all other jailbreaks of the time, it was the first truly rootless jailbreak, which means that it did not install any files in the system partition. However, because of its early release date, it did not support the Procursus-developed rootless package format that would become standard in iOS 15 and later jailbreaks (since it had not yet been invented) and thus tweak support for this jailbreak was extremely limited.