Final answer:
Phase 4 of the cardiac cycle, ventricular diastole, involves the ventricles relaxing to allow the atria to push blood into them for the next cycle.
Step-by-step explanation:
In Phase 4 of the cardiac cycle, known as ventricular diastole, the ventricles are in a state of relaxation following contraction. This phase is critical because as the ventricular muscles relax, the pressure within the ventricles decreases, allowing the pressure from the atria to push blood into the ventricles. Both the atrioventricular (tricuspid and mitral) valves are open, and the semilunar valves (pulmonary and aortic) are closed, enabling blood to flow seamlessly from the relaxed atria into the ventricles in preparation for the next cardiac cycle. This phase ensures that the heart chambers are filled with blood and ready for the initiating event of the cardiac cycle: atrial systole.