This article presents a new four-port electronic circulating duplexer (CD) that enables similar functionality to that of a four port circulator. The proposed CD is formed by connecting a quadrature hybrid (QH) and a quasi-circulating quadrature hybrid (QCQH). We derive the S-matrix of the CD by employing Mason's rule and verify the analytical results by simulations in which the 90° NRPS of the QCQH is implemented as a behavioral 8-Path NRPS. The functionality of CD enables implementation as a simultaneous transmit-receive (STR) RF front end that includes transmit (TX), antenna (ANT), self-interference cancellation (SIC) and receive (RX) dedicated ports. Measurements of a CD with a commercial QH connected to a QCQH that employs 65-nm NMOS 4-path NRPS integrated on PCB, show a 1.6 dB TX-to-ANT, less than 2 dB ANT-SIC and 2.9 dB SIC-to-RX losses over 40MHz bandwidth. TX-RX and TX-SIC isolation are also measured showing 18 and 13.5 dB respectively. A 5 dBm modulated TX signal at 970 MHz achieved a total isolation of 55 dB with active SIC injection along with a TX EVM of -39 dB.