When I paste my DSN (Data Source Name) into a browser and try to navigate to it, Sentry always returns a 404 error. I want to know why, and how to check that my DSN is valid.

A 404 is expected. A DSN is not a web page you can open in a browser. It is a configuration string that tells the SDK how to authenticate with Sentry and where to send your events, so opening it directly returns a 404.

Your DSN is still valid. To view or verify it, open your project in Sentry and go to Settings > SDK Setup > Client Keys (DSN).