Ser/Thr-kinase component of Cyclin D-CDK4 (DC) complexes that phosphorylate and inhibit members of the retinoblastoma (RB) protein family including RB1 and regulate the cell-cycle during G(1)/S transition. Phosphorylation of RB1 allows dissociation of the transcription factor E2F from the RB/E2F complexes and the subsequent transcription of E2F target genes which are responsible for the progression through the G(1) phase. Cyclin D-CDK4 complexes are major integrators of various mitogenenic and antimitogenic signals. Component of the ternary complex, Cyclin D3/CDK4/CDKN1B, required for nuclear translocation and activity of the Cyclin D-CDK4 complex. Shows transcriptional coactivator activity with ATF5 independently of CDK4.