Insulin-producing pancreatic islet β cells (β-cells) are destroyed, severely depleted or functionally impaired in diabetes. Therefore, replacing functional β-cell mass would advance clinical diabetes management. We have previously demonstrated the importance of Cdk4 in regulating β-cell mass. Cdk4-deficient mice display β-cell hypoplasia and develop diabetes, whereas β-cell hyperplasia is observed in mice expressing an active Cdk4R24C kinase. While β-cell replication appears to be the primary mechanism responsible for β-cell mass increase, considerable evidence also supports a contribution from the pancreatic ductal epithelium in generation of new β-cells. Further, while it is believed that majority of β-cells are in a state of ‘dormancy’, it is unclear if and to what extent the quiescent cells can be coaxed to participate in the β-cell regenerative response. Here, we address these queries using a model of partial pancreatectomy (PX) in Cdk4 mutant mice. To investigate the kinetics of the regeneration process precisely, we performed DNA analog-based lineage-tracing studies followed by mathematical modeling. Within a week after PX, we observed considerable proliferation of islet β-cells and ductal epithelial cells. Interestingly, the mathematical model showed that recruitment of quiescent cells into the active cell cycle promotes β-cell mass reconstitution in the Cdk4R24C pancreas. Moreover, within 24–48 hours post-PX, ductal epithelial cells expressing the transcription factor Pdx-1 dramatically increased. We also detected insulin-positive cells in the ductal epithelium along with a significant increase of islet-like cell clusters in the Cdk4R24C pancreas. We conclude that Cdk4 not only promotes β-cell replication, but also facilitates the activation of β-cell progenitors in the ductal epithelium. In addition, we show that Cdk4 controls β-cell mass by recruiting quiescent cells to enter the cell cycle. Comparing the contribution of cell proliferation and islet-like clusters to the total increase in insulin-positive cells suggests a hitherto uncharacterized large non-proliferative contribution.