Putative regulatory subunit of serine/threonine-protein phosphatase 4. Serine/threonine-protein phosphatase 4 (PP4) occurs in different assemblies of the catalytic and one or more regulatory subunits. Component of the PP4 complex PPP4C-PPP4R4 (By similarity). May be produced at very low levels due to a premature stop codon in the mRNA, leading to nonsense-mediated mRNA decay. Intron retention.