It will have some killer features and some debatable ones. This should be the killer feature. The Palm III fits fine in just about any pocket. If the Palm IV is too small, it will start getting pretty hard to write on but if you only use it to look up addresses an phone numbers in, you won't care. I think rechargeable batteries are but one more point of possible failure that can take you out of operation.
Rechargeable batteries sound just sounds like a marketing feature without real value.
I also hear that Hotsynching may be done via InfraRed instead of the cradle. That's a definite negative for me and almost bad enough to make me want to look at CE alternatives. Hotsynching via InfraRed to my PC is not ready for prime time because it is unreliable and slow. The the cradle system for Hotsynching works fine. If we have a rechargeable model only, I vote to build the Hotsync capability into the charging unit.