The aim of this paper is to analyse and frame the way in which the boundaries of human capital-intensive firms (HCIFs) are changing as a consequence of the growing importance of fourth-party logistics (4PLs). The specific relationship between these two types of modern firms is representative of the transformation of inter-firm coordination and sheds light on the resulting expansion of the boundaries of the firm. Based on the modern theory of the firm, we show that the traditional legal-contractual approaches are inadequate for investigating the strong and complex links between an HCIF and a 4PL contrary to the “new” economic-strategic approaches of the firm. The restrictive legal vision of the firm they support needs to be transcended by functional and productive recent analyses that we summarise in four propositions. Thus, the service supply chain relationship that we study is useful in laying the foundations of a broader framework on the boundaries of the firm.