Your goal is to have seperate machines w/ seperate IP addresses. This way, should your primary DNS service die, there are failsafes to fall back on.
In my opinion, you really only require two DNS servers. You can use the other IP addresses for other services / applications.
If you are keen on having more than two DNS servers, my suggestion would be an off-net backup namesevice offered by most ISP / hosting companies. Alternatively, there are also free solutions you can use (zonedit, everydns)
Out of curiosity, are you setting up your own servers, or is your hosting company providing anonymous DNS for you? If your case is that latter, then there is no point to using your IP addresses, as your hosting company will more than likely just alias these to there nameservers.