Now that Android 6.0 Marshmallow is official and the Nexus devices mostly have the new software, users with smartphones from other OEMs are eagerly awaiting the release of the upgrade for their own devices. Many of the top OEM's, including Sony have already opened up about the plan for updating devices to Marshmallow, and Sony has confirmed which phones and tablets they plan to update although at this point in time there isn't a specific release window. Although plenty of Sony's devices are already on Android 5.0 Lollipop, some of them still haven't received the update to the latest version of it, Android 5.1.1, like the Xperia Z3+ and the Xperia Z4 Tablet.
According to the most recent details from Sony's Xperia product support pages, it look like Sony has decided to forego the Android 5.1.1 Lollipop update for those devices altogether and instead are going to work diligently to bring them forward to Android 6.0 Marshmallow. As for when each device that hasn't yet been updated to 5.1.1 Lollipop will make the move to 6.0, that will likely vary region by region and depending on the carriers which those devices run on. A rumor from earlier this morning pegs the Telstra version of the Sony Xperia Z4 Tablet as potentially getting its update to Marshmallow sometime in January of 2016, so it's possible that other Xperia Z4 Tablet models will get it around the same time. It's also possible it could be sooner or later.
Following Telstra's information about the Xperia Z4 Tablet, Sony's support page lists details that mesh with that information, although other devices seem to be included as well like the Xperia Z3+, the Xperia M4 Aqua, the Xperia C4, the Xperia C5 Ultra, and the Xperia M5. Since none of these devices are officially on Android 5.1.1, it makes sense for Sony to jump straight to the newer version of Android now that it's been released. This will lead to a longer wait for users to get an updated version of Android, but in the long run they'll be on the most recent version which comes along with the new UI perks and permissions changes, as well as the security fixes.