R5 supports resolve of constants in https://github.com/hapifhir/hapi-fhir/blob/5aa760c1e290c1e36bfd85e86b0d175073940436/hapi-fhir-structures-r5/src/main/java/org/hl7/fhir/r5/hapi/fhirpath/FhirPathR5.java#L105 - whereas R4 does not -> https://github.com/hapifhir/hapi-fhir/blob/5aa760c1e290c1e36bfd85e86b0d175073940436/hapi-fhir-structures-r4/src/main/java/org/hl7/fhir/r4/hapi/fluentpath/FhirPathR4.java#L104
