Drives are added to the compatibility list when tested. Drives are tested once available... You are not going to get any guarantees before testing is done, but there is currently no reason or any form of indication that 4TB or even larger drives cannot be supported
First NetGear has to wait for a bare 4TB drive to be released. Then it has to purchase a large quantity of the drives (need to allow for disk failures) to test in multiple units (may not be easy particularly as drive availability becomes very constrained due to the ongoing effects from the Thailand floods) Testing will likely begin with the newer ReadyNAS models (as NetGear may wish to move to selling some models semi-populated or fully-populated with 4TB disks) and then later move onto the NVX. If NetGear encounters problems it will then need to work on a fix and redo its testing (possibly requiring purchase of more drives). It's an expensive process and can take a while, but they understand the importance of doing their testing thoroughly.
I can't see any reason why 4TB drives wouldn't work either, but NetGear will want to make sure they work fine before adding them to the compatibility list.