The "Cluster not found"-variant HTTP 404 is distinct from the "Index not found" message. This error message indicates that the routing layer is unable to match your URL to a cluster resource. This can be caused by a few things:
If you have confirmed that: A) the URL is correct (see Connecting to Bonsai for more information), B) the cluster has not been destroyed, and C) the cluster should be up and running, and D) you're still receiving HTTP 404 responses from the cluster, then send us an email and we'll investigate.