If you're getting a 404 error after clicking the "Edit Website" button in the website builder, simply log out of your account and log back in.
This issue is usually caused by the page being open for over 15 minutes. As a security feature in case you leave the page open, the security token that allows you to enter the website builder expires. If you are still logged in refreshing the page should grant you a new token and let you into the builder.