i'm experiencing a server failure today with Flexihostings. It's made me wonder. If I had 3rd and 4th nameservers setup to point to another server and had mirrored versions of my sites on that server would the nameservers eventually work down to the 3rd and 4th servers, and show the mirrored sites?
It depends upon what you want. Nameservers respond via round-robin, so all 4 nameservers would probably get the same amount of hits. If you don't mind traffic going to both sites at all times, It'll work out well. However, If you only want the secondary site to be active during periods of downtime, This option won't work. That said, If one nameserver is down, the client's DNS will usually go on to the next one, until it can find a working nameserver, so it would give you some of the reliability you're looking for.
But its not easy to mirror if the site is dynamic and depends heavily on database...static is no problem.
Generally you want to use some sort of off-site DNS service, and have a short TTL (Total Time to Live) set. When your site fails, change the IP to the new server in the DNS setup. After the TTL passes, most people should be directed to the backup server.