Specifically phosphorylates the activated forms of G protein- coupled receptors. GRK4-alpha can phosphorylate rhodopsin and its activity is inhibited by calmodulin; the other three isoforms do not phosphorylate rhodopsin and do not interact with calmodulin. GRK4-alpha and GRK4-gamma phosphorylate DRD3. Phosphorylates ADRB2. . This is the function of Ensembl gene identifier ENSG00000125388 (GRK4, G protein-coupled receptor kinase 4).