Both of these devices were introduced to the masses some time in the first half of last year, where they arrived with Android 5.0 Lollipop on board. It looks like the LTE (SGP771) and Wi-Fi-only (SGP712) versions of the Xperia Z4 Tablet will be getting the Android 6.0 Marshmallow update, while the Xperia Z3+ smartphone will only see the single SIM variant (E6553) get the update, but this does not mean that the dual SIM model (E6533) will miss out on it.
As Sony sells the Xperia X3+ and Xperia Z4 Tablet in the US as well in an unlocked setting via retailers, the Android 6.0 Marshmallow update for said devices will be unavailable via your mobile carrier, so do take note of that so that you need not have to keep on refreshing the software update menu on the handset.