The internet is often a dangerous place. As an innocent surfer you will never know for sure what the result of a click on a link will be. Even normally trusted sites might be infected by malware due to malvertising (see also anti-malvertising.com), bot attacks or other nasty tricks.
ZOMDir tries to minimize the probability that you visit a site that has wrong intentions. Therefor all links at ZOMDir are checked with Safebrowsing at a Glance.
At a regular base there is a content check. When a linked page doesn't load, is redirected or is probably unsafe to visit then this link will be removed automatically. When this occurs you will find in the overview of removed links the corresponding error code. In this list you might find also status code 604.
Status code 604 indicates that it is probably unsafe to follow the related link. The reason might be that link is marked as suspicious by Google Safebrowsing or that this link is marked as unreliable at the Web Of Trust.
That's all I can tell you now about this status code.
ZOMDir.com is a dynamic directory and a wiki
Everyone is able to add a link in 10 seconds
To learn more view this Slideshare presentation