How to route 404 errors for multi-site Craft? Craft looks for /templates/404 by default. What do you do if you have multiple sites?
I pondered this for a while, and then did this, which will direct us to specific entries (not templates, I don't want to create more of those):

{# We need a unique 404 for each site - just take us to the 
   right slug relative to current URL #}

{% switch currentSite.id %}

{# siteA.com #}
{% case 1 %}

    {% redirect '/my-404-slug' %}

{# siteB.com #}
{% case 2 %}

    {% redirect '/my-other-404-slug' %}

{% endswitch %}


If you want to use templates instead of entries, just use an {% include %} statement instead of {% redirect %}.