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. [G-protein-coupled receptor] + ATP = [G-protein-coupled receptor]-phosphate + ADP + H(+) Inhibited by heparin. Interacts with DRD3. Both at the cell surface and dispersed in the cytoplasm under basal conditions. Receptor stimulation results in the internalization of GRK4 to the perinuclear area, where colocalization with DRD3 is observed strongly at 5 and 15 minutes. DRD3 and GRK4 colocalize in lipid rafts of renal proximal tubule cells. Isoform 1, isoform 2, isoform 3, and isoform 4 are expressed in testis. Isoform 4 is expressed in myometrium. Palmitoylated. Belongs to the protein kinase superfamily. AGC Ser/Thr protein kinase family. GPRK subfamily.