Q.How would you interpret the equation Q = W (Eq. 4.21) for a cyclic process?

A cyclic process is one in which a thermodynamic system, after undergoing a series of changes, returns exactly to its initial state. Since the system's state variables (and hence its internal energy, a state variable) return to their starting values, the net change in internal energy over one complete cycle is always zero, $\Delta U_{cycle}=0$. Applying the first law over the full cycle then gives $Q_{net}=Q_{in}-Q_{out}=W$: the net work done by the system in one cycle exactly equals the net heat absorbed. On a P-V diagram, a cyclic process appears as a closed loop, and the area enclosed by that loop equals the magnitude of the net work done in the cycle -- if the loop is traced clockwise, the net work done is positive (net work is done BY the system); if traced anticlockwise, the net work done is negative (net work is done ON the system). This is the working principle behind every heat engine: by repeatedly cycling a working substance around a closed loop on the P-V diagram, net positive work can be continuously extracted from a net heat input, cycle after cycle.