PPP4R4 (protein phosphatase 4 regulatory subunit 4) is a regulatory subunit that forms a novel stable cytosolic complex with the catalytic subunit of protein phosphatase 4 (PP4c) 1. Unlike the scaffolding role of PP2A regulatory subunits, PPP4R4 associates specifically with PP4c but does not bridge it with other characterized PP4 regulatory subunits, representing an independent regulatory mechanism 1. The PP4c-PPP4R4 complex exhibits phosphatase activity toward fluorogenic substrates and the DNA damage marker gammaH2AX, though with lower activity than other PP4 regulatory protein combinations 1. PP4c participates in critical cellular processes including microtubule organization, DNA damage checkpoint recovery, and apoptosis regulation 1. PPP4R4 expression is dysregulated in disease contexts: it was identified as downregulated in FASN-silenced osteosarcoma cells 2, as a target of smoking-related microRNA hsa-miR-627-5p in airway epithelium 3, and associated with severe chr14 obstructive pulmonary disease (COPD) risk loci 4. These findings suggest PPP4R4 plays roles in phosphatase regulation, lung disease pathogenesis, and potentially cancer development, though direct functional characterization in disease contexts remains limited.