The clip above, from the official AeroIndia 2015 video of the Defence Research & Development Organisation (DRDO) happens to be the first public acknowledgement of the so-far classified K4 long-range underwater launched nuclear-capable ballistic missile system. The DRDO subtitles the K4, somewhat enigmatically a 'longer range' system in the video -- clearly a reference to the 750-km range K15/B05 sub-surface launched weapon that's had more than ten tests so far (and that the DRDO officially acknowledged only in 2013).
The video used in the clip above is in some dispute. Is it really the K4? It doesn't appear to be the K-15. There are no available photographs of the K4, so if this is indeed the single March 2014 test that took place secretively in the Bay of Bengal, these are the first images of the system publicly revealed.
The launch sequence is significant. Compare the clip above with the K-15/B05 system and the BrahMos under-water launched variant, the only two other known sub-surface launched weapons in test. The 'K' family of weapons intended for submarine launch were revealed first by friend and colleague, and India Today deputy editor Sandeep Unnithan in 2010. Unnithan, who has had a look at these images says he understands that the K4 is a 'fatter' missile like the Polaris, but couldn't be sure given the detail in these images.