A custom 404 error page can be invaluable in retaining traffic to your site which would have otherwise been your competitors.
In some cases it may be broken links on your site that cause a 404 error, but with so many link checking programs available (many for free) this can be easily remedied.
However, a good proportion of your traffic probably comes from search engines. If you are a well indexed site and you are redesigning your website you could be in for a nightmare situation, where many of your indexed pages become defunct.
If this is the case when a user attempts to load a page under your domain and recieves the default “Page Cannot be displayed” 404 error, it is highly unlikely they are going to attempt to access your home page by editing the url. What I can virtually guarantee, is that 99% of users will click the browsers back button and search again.