The two handsets now get firmware versions 27.3.A.0.122 and 27.3.B.0.122, thus receiving the usual Android 6.0 features - Doze mode, Google Now on Tap, granular permissions, and more.
http://www.notebookcheck.net/Sony-Xperia-C4-and-C4-Dual-get-Android-Marshmallow.174370.0.html