We demonstrate a K-band (18–27 GHz) divide-by-4 injection-locked frequency-divider (ILFD4) in 90 nm CMOS. A wideband balun is used for converting the single-ended injection signal to differential injected signals. Divide-by-4 is attained using dual linear mixer technique, i.e., dual divide-by-2. The first quadrature-phase (I/Q-phase) divide-by-2 is performed by the direct-injection to the differential switch transistors. The second divide-by-2 is carried out by the tail-injection to the I/Q-phase cross-connected four-stage differential ring oscillators. Due to the inherent divide-by-4 feature of the dual linear mixer, strong differential-I/Q divide-by-4 outputs are obtained. In addition to the first-stage frequency divider of a K-band phase-locked loop, the circuit can also be used to provide the required differential-I/Q LO signals (for I/Q modulation/demodulation) of a 28 GHz 5G transceiver. The ILFD4 consumes 7.2 mW and achieves locking range of 48.4% (12.2-20 GHz) and figure-of-merit (FOM) of 6.72 mW⁻¹, one of the best results ever reported for CMOS ILFD4s with similar operation frequency.