A nuclear deterrent should either be foregone or funded with enough boats to maintain it safely. Continuous patrol keeps the capability in the background instead of requiring a conspicuous crisis-time deployment that would raise tensions. With one submarine in deep refit, the remaining three cover patrol, training, readiness, and the possibility that another boat becomes unavailable. Four hulls are therefore the minimum for guaranteeing one submarine at sea without turning the act of deployment into an escalatory signal.
Notable Point · 1:41:21-1:42:56
Why continuous deterrence requires four submarines
A four-boat fleet keeps one submarine at sea without a crisis-time deployment that would itself raise tensions.