Why triple turrets were the practical sweet spot

Would two quadruple turrets have been better than three triple turrets?

No; triple turrets offered a more workable balance of weight, reliability, aiming, and maintainability with contemporary technology.

Quadruple turrets promise concentrated firepower and lower structural weight on paper. Crowding, aiming, shock, maintenance, and development risk eroded those savings in service. Triple mounts offered the best compromise available to 1920s and 1930s engineering. Later British and French quad-mount difficulties illustrate why concentrating another gun in one mounting could reduce practical fighting reliability.