Existing Wording
If the resource cannot be deleted because it does not exist on the server, the server SHALL return 404 (Not found). Performing this interaction on a resource that is already deleted has no effect, and should return 204. Resources that have been deleted may subsequently be brought back to life by PUTting an update to them subsequent to the deletion.

Comments
Any delete on a non-existent resource should return a 204. A delete of a non-existent resource is always successful, in the same manner as "Performing this interaction on a resource that s already deleted has no effect, and should return 204". A 404 return indicates an exception, and when considered within a transaction, would serve to kill the transaction, when in reality, all is well. If there is a need to distinguish between fine grained FHIR-specific notions of "delete but no really because I can bring this back to life", then suggest the 404 be replaced with some other success code that can be interpreted in that finer fashion without automatically throwing an excaption.

Grahame's Comments
agree. Also for security reasons. And see task #3908