You don't indicate what version of code you are running on the controllers, but in the newer releases, I know 184.108.40.206 has it and I believe that 220.127.116.11 has this feature as well; but you can set a priority on the AP's. So if there are key areas of your facility that are more important than others you can specify them to have priority to associate to the controller so that in the end if one controller failed you'd only have 4 AP's down.
The other option would be to configure H-REAP on the 16 AP's although this is more overhead and management work, it would afford you the ability to lose the controllers and main working as long as the AP's don't lose power.
Transferring Crash file from standby: Login to the Active WLC in HA.
From CLI: (Cisco Controller) >transfer upload datatype crash (Cisco
Controller) >transfer upload filename (Cisco
Controller) >transfer upload mode tftp (Cisco Controller) >transfer
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