A 404 error is a standard response that indicates:
A server is down.
It cannot find the requested content (due to file or domain removal).
The requested URL is broken.
Set one of your pages as a 404 page so that when a visitor tries to visit a page with the mentioned issues above, they will be redirected to your 404 page.
Open the campaign
Click Page Actions
Select Make This My 404 (Error) Page
Publish the page