Sony Xperia Z3+ and Z4 tablet have been approved for first firmware update. The new update will have 28.0.A.7.24 as build number. Both the devices have the current build number number 28.0.A.6.8. So you can expect to received OTA update notification soon.
Xperia Z3+ and Z4 both the device have heating issue because of Snapdragon 810 chipset, and Sony has already said that both smartphones will receive update somewhere this summer to fix the issue. This could be the update Sony was talking about, but we are not sure.