A fourth Yamato turret drives the ship toward 80,000 tons

How large would Yamato become with a fourth triple 18-inch turret while retaining its speed?

About 80,000 tons fully loaded, because the turret requires a larger hull, more structure, and additional power as well as the guns themselves.

Yamato already displaced roughly 72,000 tons fully loaded and made about 27 knots. A fourth triple turret adds about 450 tons in gun barrels before counting the rotating structure, barbette, magazines, and protection. Maintaining speed requires a longer hull and somewhat more power, so the added weight propagates through the whole design. A rough estimate places the result near 80,000 tons fully loaded. Adding one turret therefore becomes a system-wide redesign rather than a simple arithmetic increase.