Reliability, reliability with repair, and availability concepts for specific configurations of four identical element systems, including series, parallel, and multiplex systems, are developed using a continuous time Markov process. As the number of systems components increases, the Markov approach leads to complex sets of differential equations that require computer solution. The multiplex system availability, reliability with repair, and reliability are superior to that of both the parallel and series systems. Multiplexing offers the opportunity for reliability improvement in the design of nuclear safety systems.