Serine/threonine-protein kinase which, in association with cyclin D-like protein cyd-1, is required for the progression through the G1 phase of the cell cycle during postembryonic development by phosphorylating and inhibiting lin-35 and fzr-1 (PubMed:10518501, PubMed:11684669, PubMed:25562820). In complex with cyd-1, involved in sex determination during gonadogenesis by regulating the asymmetric division of the somatic gonadal precursor cell (SGP) (PubMed:16198291). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with cyd-1; the interaction is likely involved in regulating cdk-4 activity. Expression initiates during mid-embryogenesis primarily in post-proliferative hypodermal cells and neurons in the head, ventral cord and tail, then declines until hatching where it is mainly seen in seam cells. Expressed throughout larval development in several blast cell linages. In the P lineage, expression is restricted to proliferating cells, whereas it persists in somatic gonads and seam cells. Expressed in uterus and intestine and to a lesser extent in spermatheca. Arrested cell division in the G1 phase during larval development (PubMed:25562820). RNAi-mediated knockdown results in L4 stage arrest which is associated with uncoordinated movements and a protruding vulva (PubMed:10518501). Impaired cell division of P blast cells, somatic gonad precursors Z1 and Z4 and intestinal cells (PubMed:10518501). Severe defect in the proliferation of P blast cells, intestinal cells, vulva cell precursors and somatic gonad precursors (PubMed:10518501). Mesoblast M cell division is normal (PubMed:10518501). Double knockout with lin-35 rescues the cell cycle progression defect in the single cdk-4 mutants (PubMed:11684669, PubMed:25562820). Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. CDC2/CDKX subfamily.