As part of the v2-to-FHIR project we are mapping commonly used fields in key v2 messages to FHIR resources so that messages can be transformed into bundle of FHIR resources. OBX-4 is the SubID field but there is no cognate existing data element in FHIR. We suggest that a standard extension to Observation be created to allow exchange of this concept. 

While the original definition of OBX-4 used the string data type, new versions of v2 use a more complex (OG) data type which needs to be accommodated.