My source at SMC says Friday. Also said that 4.4 brings finally brings features worth talking about to the public rather than the background changes that 4.2 and 4.3 brought.
Devices that got updated to 4.3 will all be able to update to 4.4 as they have the same baseline requirements apparently. 4.4 changes top layer stuff leaving most of the behind the scenes stuff unchanged.
Also got an updated schedule for the 4.3 rollout for Xperia:
Z/ZL - end October
Z1, Z Ultra - middle November
Tablet Z, ZR - end November
SP - middle December
L, M - Jan to Feb (EOL)
T, TX, TL, V - Spring 2014 (EOL)
There is a secondary team working in parallel with the 4.4 PDK for Z1, ZU and a couple of other S800 devices that are released in early 2014, this is the same method Samsung use to get their flagships updated quicker. The secondary team will get the framework ported to S800 and get the devices feature complete, by that time the main team for flagships will be freed up to port the extras and optimise the builds. They also told me that the Z1 went through 7 major milestone builds before they got 4.2.2 running to an acceptable level which is what held up development, that was is the reason 4.3 is currently running late, usually it only takes the team two or three milestones to get to consumer grade builds, but with the Z1 it took them much longer. The source said 4.3 is better though, they are close to consumer grade software on their second build and the third "version" will start from scratch soon so they will be able to iron out the remaining bugs and hopefully make it by the middle of November and stick to their internal timetable.