Is this API FHIR R4 compliant?

Yes. The implementation is documented in the Capability Statement available at the /metadata endpoint.

Do I need an authorization token to retrieve the Capability Statement?

No. Per the FHIR specification, the /metadata endpoint does NOT require authorization.

Is HTTPS required for all transaction?

Yes, all transactions with the service MUST use TLS.

Are any of the resources based on profiles?

Yes, the Task resource is profiled to constrain the intent property and to require a contained Patient resource holding the demographic data for patient matching.

Does the FHIR server use particular coding systems?

The input and output property types of the Task resource are described using value sets defined for this implementation.

What types of clinical data is returned by Clinical Document Collector?

This depends on the capabilities of the data providers, but the documents will typically be Consolidated CDA documents in XML format.