Protein phosphatase 4 (PP4) plays a crucial role in various cellular pathways, and its proper regulation is essential for optimal phosphorylation dynamics and supporting critical cellular pathways, including DNA double-strand break repair. A previously identified PP4 regulator, termed PP4 inhibitory protein (PP4IP), has been implicated in modulating PP4 activity. However, the biochemical properties of PP4IP and the details of its relationship with the PP4 complex remain poorly understood. In this study, we provide insights into the biochemical characteristics of PP4IP and investigate its physical interaction with PP4. Contrary to a previous report, we were not able to detect any ribonuclease activity in PP4IP purified from E. coli, insect, or mammalian cells. On the other hand, our findings reveal that PP4IP dimerization is required for its interaction with the PP4 complex, and that PP4R3α is a key mediator of this interaction. This study provides a clear understanding of the mechanism underlying PP4-PP4IP complex formation.