If you've come across adfs4.sts.altareturn.com and wondered whether it's safe, the short answer is that there's not enough working here to trust it. The page doesn't load anything useful — just a 404 error. That alone is unusual for any kind of website, especially one that looks like it's meant to handle authentication (the name suggests it's a Microsoft Active Directory Federation Services endpoint).
Most legitimate technical services have some form of working interface or at least a redirect. This one doesn't. There's no contact information, no company name, and no way to verify who set it up. The SSL certificate is valid, but the server still accepts outdated encryption that major browsers stopped supporting years ago.
When a site is this bare, it could be a test server that was abandoned, a misconfigured internal tool, or something set up to harvest credentials if it ever goes live. Because we can't see any real content or identity, the safest approach is to treat it as untrustworthy. There are no adfs4.sts.altareturn.com reviews or any public record of the site working properly, so assume it's not a service you should rely on or visit.