For those in the states with the T-Mobile branded Sony Xperia Z, your last update was to Android 4.2 back in the fourth quarter. According to T-Mobile's software support page, the carrier does anticipate updating the device to Android 4.4 once it becomes available. Of course, the update will have to pass T-Mobile's internal testing.
Unfortunately, no precise date has for the update has been announced by the Japanese manufacturer, but as soon as Sony releases a date, or starts pushing it out, we will let you know.
source: XperiaBlog via AndroidCentral