Why were fourth-rate ships of the line phased out?

Fourth-rate ships of the line were primarily phased out due to their firepower limitations. These ships typically carried fewer than 64 guns, which made them less effective in battle compared to their higher-rated counterparts that possessed more formidable armament. In naval warfare, the ability to deliver significant firepower was crucial for engaging enemy vessels and ensuring dominance in naval engagements. As naval tactics and ship design evolved, the need for vessels with greater firepower became evident, leading to the decline of fourth-rate ships that could not meet these emerging requirements.

While factors like design and crew size contributed to the overall effectiveness of a ship, the primary reason for the phasing out of fourth-rate vessels was their inability to compete in terms of sheer firepower, rendering them less valuable in the context of fleet actions and naval strategy.