A 404 Error means that the browser was able to communicate with Solr, but Solr could not find what was requested.
In the case of a SearchStax Solr deployment, the 404 Error usually means that you sent a query to a collection that did not exist. This could be due to a typo in the collection name, or to the fact that the collection is simply missing.
In one case, someone sent in queries that demanded &rows=100000000… one hundred million rows of output. Solr could not allocate that much space, and replied with a 404 error.