Thankfully that's changed today, since the KitKat update for the Xperia Z1s has finally began its rollout. Furthermore, it will take the handset directly to Android 4.4.4, which is currently the latest version of Google's mobile operating system.
To update, unfortunately you need a computer with Sony's PC Companion software installed, as this isn't an over-the-air rollout. Your phone will probably notify you about the update, but it will tell you to use a PC for the actual procedure.
After applying the update, you'll be on build number 14.4.B.0.37. The new software brings with it a new launcher, new themes, a customized keyboard, transparent navigation and status bars, white status bar icons, a bunch of upgraded Sony apps, a new boot animation, and a more responsive touchscreen.