According to the grapevine Sony had no plans of upgrading Sony Xperia C4 to Android 5.1 Lollipop. In October 2015 the software support pages of Xperia C4, C5 Ultra and other phones in the same range coming with Android 5.0.2 had confirmed that the company would skip Android 5.1 Lollipop for the C range of mobile phones and directly upgrade to Marshmallow but the plan seems to be altered now.
Although users who wanted to upgrade their Sony Xperia C4 to the next Android version are a bit disappointed because they were thinking that they will get Android 6.0 Marshmallow but there seem to be initial glitches and the Japanese manufacturer has decided that instead of waiting for the glitch to go away, they will move onto the next-best available version for its users. Both C4 and C4 Dual are getting the updates. Build number 27.2.A.0.155 is for Xperia C4 and 27.2.B.0.155 is for Xperia C4 Dual, according to this Xperia blog.
The upgrade is being released in different phases. If your phone hasn’t received a notification you can manually checked by going to
Settings >> About Phone >> System Updates >> Check
It may be a few weeks before your phone gets the update.
Android 5.1 Lollipop comes with significant updates. It allows apps to load faster. The “okay Google” snag that was bringing many phones to the silent mode and this seems to have been sorted out in the 5.1 version. Here’s a video that explains what’s new in Android 5.1 Lollipop.