Using Login-as feature redirects the user to a url that gives a 404 error.

This error occurs because the Sales App (Classic type) does not have a valid default landing tab configured. When an administrator uses Login-as, Salesforce attempts to redirect to the app's default landing page — if no tab is set, the redirect results in a 404 error.

To resolve the error, please follow these steps:

1. From Setup, enter App Manager in the Quick Find box, and select App Manager.
2. Find the Sales App (App Type: Classic). Click the dropdown arrow and select Edit .
3. Select a tab from the Default Landing Tab dropdown. Then click Save.