To prevent data from being accessed by unauthorized users, it is necessary for Credit Card Transaction Processing(CCTP) systems to use multilevel secure database management systems to control concurrent execution among multiple transactions. In CCTP systems, analytical transactions as well as mission critical transactions are executed concurrently, which causes difficulties in using traditional secure real-time transaction management schemes in the systems. In this paper, we propose a read-down secure single snapshot scheme(RS4) that is devised for secure real-time transaction management. By maintaining a snapshot as well as the working database, RS4 blocks covert-channels without causing a so-called priority inversion phenomenon. We introduce the process of RS4 protocol with some examples, present proofs of the devised protocol, and then evaluate performance gains by means of a simulation method.