List of dependants can be found at https://packagist.org/packages/serialization/serialization/dependents
The release notes for 4.0.0 can be found at https://github.com/wmde/Serialization/releases/tag/4.0.0
As of 18/08/2018 the components that need to be updated are as follows:
 composer library: data-values/serialization
 composer library: wikibase/data-model-serialization
 composer library: wikibase/internal-serialization
 extension: WikibaseLexeme
 extension: WikibaseMediaInfo
 extension: WikibaseQuality
 extension: WikibaseQualityConstraints
 wmf: mediawiki-vendor
**Impact & priority**
Currently there does not seem to be any development blocked on using the new version so this would be a low priority until such a task that is blocked by this appears.
TODO: check to see if all of these need to actually depend on this library
TODO: sub tasks?
- All listed components can be installed with serialization/serialization 4.0.0