> used in a style - if one is creating an overview map .

yes, I've added a check for this in --check-styles

> I see 4b functioning as an essential 'container' holding the various 4a
> polygons together ( its often used in NT maps to represent the sea/ocean).
> Interestingly, the error message caused by the 'locked flag,' is less
> severe when adding a 4b polygon.

Please try r2627, this adds now the 4a background polygon.

Gerd