Why four major ships create a usable rotation

Would Italy have been better off with four helicopter cruisers instead of three?

Yes; four improve the chance of keeping one deployed and another ready, the same rotation logic that favors several LHDs and landing ships.

Four Italian helicopter cruisers would be better than three because availability matters as much as nominal strength. An ideal fleet would buy at least six of an important type, though cost and accepted risk usually prevent that. For Britain, the equivalent proposal is three LHDs to replace Albion and Bulwark and roughly six landing ships to replace the existing logistic lift. Combined with the two Queen Elizabeth-class carriers, that structure would normally keep two aviation ships and two large dock ships available. A unique or two-ship class can look impressive on paper yet lose its capability whenever a ship enters maintenance.