There is no documentation for 4.15 because it was some thing iPLanet never "supported". However it can be done, you just have to integrate it yourself.
But, the access services (IMAP, POP, MSHTTPD, STORE) in NMS 4.15 are the same (basically) as in iMS 5. So, theoretically all you'd have to integrate/customize your self would be the part that monitors the MTA.
The reson it's "supported" in iMS is because iPlanet is providing the scripts/agents to integrate it with Sun Cluster or Veritas.
Hope this helps.
-Chris