On May 15th, Unity Cloud Build will be removing support for Unity 4.x.
There are a very small number of projects still using Unity 4.x in Cloud Build, and those users will be notified of the upcoming change directly via email as well. At the time of removal, any build targets you have configured to use Unity 4.x will be disabled and will need to be manually updated to target a newer version of Unity.
In the future, versions of Unity will be targeted for removal from Unity Cloud Build 1 year after Sustained Engineering stops supporting development of that version of the editor. Before any Unity version is removed, we will post a notice here 30 days in advance and notify any users directly affected by the change via email.