I'm quite surprised that they kept this project under the wraps till now. The names changes must have helped (as the article states) because I do remember that there was confusion over Sagarika..and K 15 and so forth a few years ago in the defense forums/circles.
What really impresses me is the height reduction achieved in the K-4 missile. its just 10-12 meters for 3500-5000 km range. For SLBMs height is a major concern as they need to be kept in vertical positions inside the sub's hull.
The current US trident 2 has a height of around ~15 meters with a range of ~12000km. If India manages to improve the K SLBM series to a range of say...~9000 km for no height gain(within 15 meters at least) then it would surely prove to be a very efficient second strike weapon, as the ocean area the sub could hide in becomes exponentially large with even a modest increase in the Missile Range.
I'm also very interested in the 4 meter, Aircraft launched version. Even with a 200 km range(as of now) its hypersonic speed would play havoc with missile defense systems, for both sea and land based targets.
If that is improved to..say around ~500-600 kms then it would be the best standoff weapon in its class. Even right now..if the claims are indeed true...the air launched version is probably the only one in its class in the world afaik. Could be vital for taking out air defenses (SEAD operations) and for hitting high value; time critical targets...not to mention Carrier battle groups and other naval shipping.