All clients have access to R4 (including global regions where FHIR has been made available), though not all clients have implemented apps, so they would need to go through an onboarding process.
Do you intend to go through the validation process as a code Program member [1]? This process is for both provider and system apps!
Our public sandbox [2] is where you should be testing your application. This domain is a model-experience, production domain used for testing, so it has no PHI/PII.
-Aaron (Oracle Cerner)