Today Sony rolled android 4.4.4 KitKat update in form of 14.4.A.0.108 firmware for Xperia Z1 Compact and same firmware has been certified for Xperia Z1 and Xperia Z Ultra. Sony Xperia Z1s is still on Android 4.3 when it was launched at CES 2014 and now a new firmware 14.4.B.0.20 firmware has been certified for it on PTCRB.
14.4.B.0.20 firmware has same naming convention like 14.4.X.X.XX for Xperia Z1, Z Ultra and Z1 C Android 4.4.4 KitKat so probably it is the new KitKat firmware update for Xperia Z1s T-Mobile US. At present Xperia Z1s is on android 4.3 14.3.D.0.183 firmware. This will be major update for Xperia Z1s users.
Xperia Z1s 14.4.B.0.20 firmware
Below you can see 14.4.B.0.20 firmware details for Xperia Z1s C6916 handset, certified on PTCRB.
Android version for 14.4.B.0.20 firmware update is still uncertain but we think it is Android 4.4.4 now.
We will keep you posted as soon as 14.4.B.0.20 firmware hits Xperia Z1s C6916. Let us know in case you get this update in meantime. Drop your comments below.