Currently running the latest version of GroupWise (18.1.1) on SLES 12 SP3. Is SP4 supported? Any known issues? I've been looking around at documentation and don't seem to find any.
First and foremost... you are not running the latest. Build 481 was removed due to a serious bug. It was replaced with build 483. I recommend you patch your GMS server as soon as possible.
With regards to the upgrade... no issues known going from SLES 12 SP3 to SP4.