Back in October of last year, Sony told us that six of its devices would not receive updates to Android 5.1 Lollipop, instead jumping from 5.0 straight to 6.0 (Marshmallow). The Xperia C4 was on that list, so obviously we were expecting to see it tasting Marshmallow soon.
Now though there's an update out for it (as well as the Xperia C4 Dual), but it's not Android 6.0. Instead, it's Android 5.1 Lollipop, a version these models shouldn't have received. Odd are Sony's ways sometimes.
Anyway, this is a new Android version, even if it's nowhere near as big as 6.0. The rollout is currently underway for five of the six C4/C4 Dual versions out there (with only the E5343 being excluded for now).
Of course the update will reach you over-the-air, probably within the next few days (as these things are usually staged). After having installed the new release, you'll be on build number 27.2.A.0.155 if you have the C4, or 27.2.B.0.155 for the C4 Dual.