<p>Here we determine reliability measures of a parallel cold standby system of four units by categorizing the units equally as phase-I (called main units) and phase-II units (called duplicated units). Initially system works with phase-I units (two) in parallel and other two phase-II units are kept in cold standby. The units are identical within each phase but are non-identical phase wise. The phase-II units become operative simultaneously at the failure of a single phase-I unit and so this is called conditional priority for operation to phase-II units. The repair activities are handled at a single repair facility and after repair each unit work as fresh. The system is analyzed using the concepts of semi-Markov process. The expressions for MTSF, availability and profit function are derived in steady state using regenerative point techniques. The behaviour of these reliability measures through graphs has been observed for particular values of the failure and repair rates. The drainage system can be considered as one the applications for the present work.</p>