You would use the IPAM v4.0 poller on an existing AP if you were running other modules on your poller than were not using the new Core version. Michal mentioned that currently only NPM 10.5 and IPAM 4.0 uses the new Core.
The only two scenarios I can think of that you would use the poller install is:
1) If you didn't have NPM at all
2) If you didn't want to upgrade NPM to 10.5 before upgrading or installing IPAM 4.0