pcp 4 breaks API, and thus is really inappropriate for an update to a stable Fedora release. E. g. it breaks Cockpit's build: https://apps.fedoraproject.org/koschei/package/cockpit?collection=f26
This was fixed upstream in https://github.com/cockpit-project/cockpit/pull/8675 (this also explains some of the API changes that Cockpit use), but I'm sure these aren't the only ones.
Also, I wonder: How can it be that the Fedora policy allows to update API-changing libraries into stable releases without checking/porting all API consumers first? And even then this breaks third-party code that is running there. It seems this already went to stable, so some things are broken now..