Hello,
If your metric its really working as well, you can create a segment that: your "eVar" does not contain "your domain" the operator: "Then After 1 Page view" Entry page=/404. Do not forget to choose the option: "Only before the sequence". There's an example:
So, after that you can go to workspace and use your segment, with your eVar, that you still need to filter the data, using "Does not contain: mycompany.com" (This is important). To have a better comparison, you can use the metric "Ocurrences" in the side:
As you can see there's no relevance in external links to 404 error in my company website.
Regards,
Mauricio Shoji