About K-4 , K-15 and Arihant submarine, it's a bit baffling.
Taking reference from the website most Indians consider reliable source the lightest and smallest diameter Indian nuclear warhead may be the one on Agni-1 which has a blast yield of 40-50 kt. That's the one most probably used on K-15.
https://missilethreat.csis.org/missile/agni-1/
On the higher side ,the Agni-4 and Agni-5 warheads are 1-1.3 meters wide and 3 metes long. That's the warhead most probably used on K-4 and has a blast yield of 150 KT.
So an Arihant submarine armed with four K-4 will have a total destructive power of about 600 KT or four warheads of 150 KT each.
The same for an Arihant armed with twelve K-15
With twelve warheads of 50 KT each.
That's a very under gunned second strike capability of just 600 KT?
Neither K-4 or K-15 are MIRV capable as too narrow.