Why a fourth Gearing turret was not a simple experiment

Why did the United States Navy not test a four-turret layout on a Gearing under construction?

Because the extra mounting would require major structural and internal redesign, not a simple deck-level alteration.

A Gearing's hull subdivisions and framing could prevent a fourth turret from fitting even while the ship was under construction. The relevant comparison is between mountings that extend between decks and those installed on the upper deck. Battle-, Daring-, Tribal-, and S-class examples show why the choice of gun mounting can require changes to the supporting structure. A light system can sometimes be fitted with limited work, while a heavier installation can demand extensive reframing. Completing a destroyer for anti-submarine work or to its existing design could therefore be more practical than making it a four-turret experiment.