In a $P$-$V$ indicator diagram:

1. Process I is parallel to the $P$-axis, meaning the volume is constant. Therefore, it is an isochoric process ($P \rightarrow c$).
2. Process IV is parallel to the $V$-axis, meaning the pressure is constant. Therefore, it is an isobaric process ($S \rightarrow b$).
3. Processes II and III are curves representing expansion. The slope of an adiabatic curve ($dp/dV = -\gamma P/V$) is $\gamma$ times steeper than that of an isothermal curve ($dp/dV = -P/V$). Since curve II is steeper than curve III, Process II is adiabatic ($Q \rightarrow a$) and Process III is isothermal ($R \rightarrow d$).

Thus, the correct match is $P \rightarrow c, Q \rightarrow a, R \rightarrow d, S \rightarrow b$.