FHIR R4 and FHIR R5: Normative Resources, Regulatory Mandates, and Architectural Differences

HL7 FHIR R4 (Release 4, v4.0.1) is the fourth major release of the Fast Healthcare Interoperability Resources standard published by Health Level Seven International (HL7) and the first version to introduce ANSI-approved Normative resources. Across the United States and international health informatics ecosystems, HL7 FHIR R4 serves as the legally mandated regulatory baseline for certified electronic health record (EHR) APIs, payer data exchange, and clinical research data pipelines, while FHIR R5 (v5.0.0) introduces expanded resource structures and topic-based event subscriptions for next-generation implementations.

What Is FHIR R4?

Published initially in December 2018 with its landmark technical correction (v4.0.1) released on October 30, 2019, FHIR Release 4 marked the transition of FHIR from a Draft Standard for Trial Use (DSTU) and Standard for Trial Use (STU) into an ANSI-accredited normative specification. Under HL7 FHIR maturity rules (the FHIR Maturity Model, FMM 0 through Normative), resources and infrastructure designated as Normative are locked for backward compatibility: future FHIR releases cannot introduce breaking structural changes without a formal multi-release deprecation process.

In FHIR R4 (v4.0.1), the following foundational components achieved Normative status:

• Core Infrastructure and Exchange: The base Resource and DomainResource definitions, XML and JSON serialization formats, the RESTful FHIR API specification (search, CRUD operations, history, and bundles), datatypes, and Terminology Module resources (CodeSystem, ValueSet, and ConceptMap).
• Core Administrative and Clinical Resources: The Patient and Observation resources, which anchor demographic identity and diagnostic, vital sign, and laboratory measurements across nearly all clinical and translational research workflows.

Why FHIR R4 Remains the Regulatory Baseline

In the United States, federal health IT regulations explicitly bind certification and compliance to HL7 FHIR Release 4.0.1 rather than newer trial-use releases:

• ONC Cures Act Final Rule and HTI-1 (45 CFR § 170.215 and § 170.315(g)(10)): The Office of the National Coordinator for Health Information Technology (ASTP/ONC) requires Certified Health IT modules to expose standardized APIs for patient and population services using FHIR R4 (v4.0.1), profiled by the HL7 FHIR US Core Implementation Guide (US Core STU 3.1.1 through US Core 6.1.0), SMART App Launch (v1.0.0 / v2.0.0), and FHIR Bulk Data Access ($export).
• CMS Interoperability and Prior Authorization Rules (CMS-0057-F): The Centers for Medicare & Medicaid Services mandates FHIR R4 APIs—incorporating HL7 Da Vinci Implementation Guides for Patient Access, Provider Access, Payer-to-Payer Data Exchange, and Prior Authorization Support (PAS)—across Medicare Advantage organizations, state Medicaid/CHIP programs, and Qualified Health Plan issuers.

FHIR R4 vs. FHIR R5: Key Architectural Differences

HL7 published FHIR Release 5 (v5.0.0) on March 26, 2023, as a Trial Use release designed to refine clinical workflow modeling, expand research capabilities, and standardize event-driven architecture before the upcoming normative R6 cycle. Key technical differences between FHIR R4 and FHIR R5 include:

Dimension	FHIR R4 (v4.0.1)	FHIR R5 (v5.0.0)
Release Date & Status	October 30, 2019 (v4.0.1); First Normative + STU release	March 26, 2023 (v5.0.0); Trial Use (STU) increment
Total Resource Count	145 resource types (plus intermediate R4B / v4.3.0 adding evidence/citation resources)	157 resource types with refined workflow and supply chain granularity
Event Subscriptions	Query-criteria-based Subscription resource evaluating server-side search strings	Topic-based publish/subscribe architecture using SubscriptionTopic, Subscription, and SubscriptionStatus (backported to R4 via the HL7 Subscriptions R5 Backport IG)
Clinical Research Resources	Basic administrative ResearchStudy and ResearchSubject structures	Redesigned ResearchStudy and ResearchSubject supporting study arms, cohorts, objective/outcome definitions, longitudinal subject state milestones, and structured eligibility criteria
Encounter & Device Modeling	Single Encounter resource tracking status/location history inline; DeviceUseStatement	Separated EncounterHistory resource to prevent unbounded payload growth; renamed and expanded DeviceUsage, InventoryItem, and FormularyItem

Handling R4 and R5 Coexistence in Clinical Research Informatics

Because production EHR vendor endpoints operate on FHIR R4 while emerging clinical trial protocol definitions (such as the HL7 Vulcan UDP and CDISC-to-FHIR mappings) leverage FHIR R5 or R4B evidence-based medicine resources (Evidence, EvidenceVariable, Citation), research data engineering teams frequently implement cross-version interoperability strategies:

1. HL7 Cross-Version Extensions and Backport IGs: When an R4 system needs an R5 capability—most notably topic-based notifications or new data elements—HL7 defines standard Cross-Version Extensions (http://hl7.org/fhir/5.0/StructureDefinition/extension-...) and formal backport implementation guides (such as the FHIR R5 Subscriptions Backport IG for R4).
2. FHIR StructureMap and ConceptMap Pipelines: Data integration engines transform R4 US Core EHR extracts into R5 study repositories using formal FHIR Mapping Language (StructureMap) definitions, preserving provenance via Provenance resources during version conversion.

Frequently Asked Questions

What is FHIR R4?

FHIR R4 (Release 4, specifically v4.0.1 published in October 2019) is the fourth major release of the HL7 Fast Healthcare Interoperability Resources standard. It is the first FHIR version with ANSI-approved Normative components—including Patient, Observation, and the RESTful API—and serves as the federally mandated baseline for ONC Certified Health IT and CMS interoperability APIs.

What is the main difference between FHIR R4 and FHIR R5?

FHIR R4 (v4.0.1) is the production regulatory baseline with 145 resources, whereas FHIR R5 (v5.0.0, March 2023) is a Trial Use release expanding the catalog to 157 resources, introducing topic-based event notifications (SubscriptionTopic), separating EncounterHistory, and substantially upgrading ResearchStudy and ResearchSubject for clinical trials.

Sources

• HL7 International: FHIR Release 4 (v4.0.1) Normative Specification (https://hl7.org/fhir/R4/) and FHIR Release 5 (v5.0.0) Specification (https://hl7.org/fhir/R5/).
• Office of the National Coordinator for Health Information Technology (ASTP/ONC): 45 CFR § 170.215 Application Programming Interfaces and § 170.315(g)(10) Standardized API for Patient and Population Services.
• Centers for Medicare & Medicaid Services (CMS): CMS Interoperability and Prior Authorization Final Rule (CMS-0057-F).