Hi Timothy, welcome to the Community!
Sourcetree doesn't require a registration, and doesn't have a trial period anymore as it's already free, you can just download it from the website, go through the installation set up (which will ask you to register with your Bitbucket or Atlassian account, but this is a different registration that what you mentioned in your message) and then you can just use it normally.
If the application still leads you to that 404 message, can you let us know what version are you running?
Supported Platforms macOS Windows We recently introduced support for additional hosting services such as GitHub Enterprise, GitLab (Cloud, Community Edition, Enterprise Edition), and...
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