Interlegal reasoning refers to the practices and methods for making good on the expectation that, where norms and arguments sourced in more than one legality are applicable to the same issue, they should all be given due consideration. Interlegal reasoning is thus part of a requirement of rational justification in authoritative decision-making under such circumstances. However, it interacts with other, potentially counterposed requirements. The question, therefore, concerns how to organise and present an appropriate structure to consider the diverse and counterposed claims of legal participants. Interface doctrines refer to a type of legal doctrines, or standards, enshrining mandatory considerations, developing as recurring argumentative patterns that structure interlegal balancing. This chapter briefly examines the fourth instance formula as an example of these interface doctrines, as it is widespread across the realms of international law. Moreover, it exemplifies a shift from dualism to interlegal reasoning.