This paper presents a novel design of quad-band rat-race couplers (RRCs) based on a modified T-shaped structure (MTSS). Two coupled lines are incorporated into the conventional dual-band T-shaped structure to create the MTSS, making it equivalent to a quarter-wavelength transmission line at four operating bands. The ABCD matrix method is applied to analyze the quad-band MTSS, deriving closed-form equations for calculating parameters of the structure. Then, a conventional RRC is separated into quarter-wavelength transmission lines, which are replaced by the proposed MTSS. To validate the design method, a quad-band RRC operating at 0.6, 1.196, 1.853, and 2.45 GHz is designed, fabricated, and tested. Simulation and experimental results demonstrate excellent agreement with theoretical prediction. The measured results show that input return loss exceeds 21.75 dB, isolation is greater than 21.6 dB, and insertion loss is less than 4.4 dB at all four operating frequencies. Phase deviations observed from the measured in-phase and out-phase responses are within 0o ± 4.6o and 180o ± 6.7o, respectively. Compared to most previously reported quad-band RRCs, the proposed circuit features a simple design and offers superior performance in isolation, return loss, and insertion loss.