You can continue to use a Media URL that PowerPress does not recommend. It's a warning message, you can ignore it.
The 404 Error means that your server is unable to retrieve the URL you entered for what ever reason (there can be many, including incorrect DNS settings, a Firewall, new traffic policy by your web hosting, etc...). If other URLs work, but that particular URL doesn't then that should be a red flag for you that something is wrong with the URL.
If the 404 Error message is incorrect, simply DO NOT use the 'Verify' button and just save your post as normal by specifying the file size and duration upon save. The value entered will save if you manually set the file size and duration information. (If you tell PowerPress to detect this information and it has a 404 error, that will be why it does not save).