India's fourth nuclear-powered ballistic missile submarine (SSBN), codenamed S4, has begun sea trials from the Shipbuilding Centre (SBC) in Visakhapatnam port, marking a key step in strengthening India's strategic deterrence. As the final Arihant class SSBN, it can carry eight K-4 nuclear missiles with over 3,500 km range, significantly enhancing India’s assured second strike capability.