Specifically phosphorylates the activated forms of G protein-coupled receptors. Plays an important role in the regulation of renal sodium handling and blood pressure. [G-protein-coupled receptor] + ATP = [G-protein-coupled receptor]-phosphate + ADP + H(+) Inhibited by heparin. Interacts with DRD3. Isoform GRK4A is expressed in testis. Isoform GRK4B is heterogeneously distributed in the kidney, with 20-fold enrichment in the outer medulla. Has a widespread but low level of expression in tissues other than testis. Palmitoylated. Belongs to the protein kinase superfamily. AGC Ser/Thr protein kinase family. GPRK subfamily.