A deduplication ratio is calculated by dividing the original size of the data by the size after deduplication. In this case, the original data is 20 GB and is reduced to 4 GB after deduplication. Thus, the deduplication ratio is 20 GB divided by 4 GB, which equals 5:1.

The content authenticity feature of CAS ensures the validity of stored content. This is achieved by generating a unique content address for each object and validating this address regularly. This process helps maintain the integrity and authenticity of the content by ensuring that the content matches its unique address, effectively preventing tampering or corruption.

The key metric that should be used when updating the corporate disaster recovery plan to minimize data loss is the Recovery Point Objective (RPO). RPO defines the maximum acceptable amount of data loss measured in time before a disaster occurs. By aiming for a lower RPO, organizations can ensure that they would lose less data in the event of a disaster.

Graceful degradation refers to the design concept where a system continues to function in the presence of partial system failures, albeit in a reduced capacity. This implies that when some modules are unavailable, the entire application doesn't go down, but rather continues to operate with limited functionality, maintaining service continuity to some extent. Thus, the statement 'Some modules are unavailable, but the entire application has not been brought down' correctly describes graceful degradation.