The 'customer_contact' with snc_external role will see the 404 on the CSM portal on navigating to certain pages even though there are no user criteria attached to the portal page.
Release or Environment
If the portal page is not configured to apply any user criteria and the page is not a public page, the non-internal users(customer_contact) are not able to access the page.
To resolve the issue, add the user criteria that apply for both the snc_internal and snc_external role so that the portal page will be shown to both the internal or external users. Another way is to make the page public by checking the 'public' checkbox to true.