Sea Lord roles after rank and title changes

Why was only the First Sea Lord a four-star officer, and what happened to the Second and Third Sea Lords?

The Second Sea Lord still led personnel matters, while the Third Sea Lord's materiel role became the lower-ranked Controller of the Navy.

The Royal Navy still had a Second Sea Lord at the time of the discussion. That office oversaw recruiting, training, welfare, diversity, sport and support to naval families. The Third Sea Lord title disappeared in 1965, but its ship and equipment responsibilities continued through the Controller of the Navy. By 1996 the Controller had fallen to rear-admiral rank, while the smaller service retained only one four-star Sea Lord. Institutional functions can survive even when historic titles and ranks disappear.