IFC4 vs. IFC2x3
Models exported/created according to IFC2x3 cannot be guaranteed to open directly as IFC4 and vice versa. This is because IFC4 is not only an extension of IFC2x3 but also contains modified and enriched existing entities and lacks some obsolete and deprecated IFC2x3 entities.
From code point of view (in both early and late binding) it is possible that software packages handling a small subset of IFC2x3 can work without any change on IFC4 files. This is achieved for the subset of the IFC2x3 Coordination View V2.0, it is however not guaranteed outside this scope.
Be aware that even when everything seems to work for IFC4 it can be the result is incomplete as extra relations could be required.