I don't know if there's an official definition, but I'd consider the Delta IV and Delta IV Heavy to be separate launch vehicles in the same family, akin to the

I don't know if there's an official definition, but I'd consider the Delta IV and Delta IV Heavy to be separate launch vehicles in the same family, akin to the Falcon 9 and Falcon Heavy. Whereas with the single-stick Delta IV Medium, the variations like 4m vs 5m fairings are just different configurations.